In a recent paper Maserati et al. (1986) demonstrated that the XO male reported by Forabosco et al. (1977) has Yp material translocated onto a chromosome 18 and that he is monosomic for a portion of the short arm of chromosome 18. In fact the patient has several signs of the 18p-syndrome. As pointed out by Maserati et al. (1986) there is increasing evidence that the XO condition may not exist but is either due to hidden or undetected mosaicism or to the translocation of Yp material which can be detected by revealing the presence of specific Y-DNA sequences, or cytogenetically as in the case reported by Schempp et al. (1985). There are two cases of XO males that have been studied by us which are likely to be further instances of unbalanced Yp/ autosome translocations resulting in severe phenotypic disturbances. The first case was reported by Fraccaro et al. (1966) and is referred to as B.N. in that paper in which the reader can find photographs of the patient aged 6 years. He was born on January 6, 1956. An intracranial hemorrhage and hypospadias with urethra opening at the base of a small penis were noted at birth. Operative reconstructions of the hypospadias were carried out at 3 and 6 years of age. Psychomotor development was retarded, and at the age of 61/2 years the following additional observations were made: mental retardation (IQ 58), shuffling gait, slightly impaired muscular coordination, and peculiar facial characteristics. A chromosome analysis of two blood cultures and of fibroblasts from one skin biopsy and one testis biopsy revealed a 45,X chromosome constitution. In 1963 the patient was operated upon because of sclerosis of the bladder neck. He also suffered from repeated infections of the urinary tract and developed a chronic and progressive pyelonephritis with uremia. Moderate development of the secondary sex characteristics was noted in 1970. On the same occasion hypertension was diagnosed. In 1971 he had epileptic seizures of the grand real type and the electroencephalogram showed abnormal features characteristic of epilepsia. He was treated with anti-epileptic drugs but had several seizures over the years despite this. He died in November 1975 due to ventricular fibrillation probably caused by hyperkalemia which developed after anesthesia for cystoscopy and intravenous pyelography. Autopsy was denied by the parents. The second case was reported by Lo Curto et al. (1974). The patient was born on March 19, 1971 and the clinical description in the paper was based on examinations made at 7 months of age (Fig. 1 in the paper). The head was relatively large and he had large ears and a broad nose flattened at the bridge. The hands and feet were short and broad. There was a single bilateral transverse palmar crease. The abdomen was prominent and there was a small umbilical hernia and bilateral inguinal hernias extending into the scrotum. The gonads, normal in size, were in the scrotum. There were several disturbances of the skeletal system which led to Lo Curto et al. (1974) to conclude that "the patient is affected with a form of 'chondroosteodysplasia' which we were unable to define more precisely". The karyotype was consistently 45,X in blood cultures and in flbroblasts from one testicular biopsy and one skin biopsy. We have re-examined some of the old slides with the same results. The tiny, fluorescent body seen at the lower right end of Fig. 2 in the paper of Lo Curto et al. (1974) was not composed of chromosome material. The patient was reexamined in 1972 when he was i year and 7 months old and his mental development was still that of a 4-month-old child. Shortly after the patient was reported to have died in Sicily and no further information is available. We conclude that is very likely that both of these patients were carriers of an unbalanced Y/autosome translocation which we are unable to identify retrospectively on the basis of their phenotypic appearance alone. This strengthens the point made by Maserati et al. (1986), namely that all cases of "XO males" are probably either hidden mosaics or carry Y-chromosome material (and the male-determining factor(s)). The only exception known to us seems to be that of the male reported by de la Chapelle et al. (1986) who was negative with six single-copy DNA (Taql) , two repeated Y-specific (HaeIII) probes, and 45,X in blood and skin fibroblast cultures.