In patients with biotinidase deficiency, an autosomal recessive condition (McKusick 253260) (Wolf et al 1985), skin rashes, alopecia, seizures, hypotonia, developmental regression and ataxia occur usually after the age of 3 months. Additionally, seborrhoeic or atopic dermatitis (eczema) are common symptoms in untreated phenylketonuria patients (PKU) (McKusick 261600), and Burton et al (1975) have found that there is an abnormality in their sebaceous gland function. We therefore measured biotinidase activity in plasma of PKU children under dietary treatment . . . . . . Fifty-three patients with classic phenylketonuria were classified into two groups according to their annual mean plasma phenylalanine concentration (Phe mean). Group A (n--27), mean age 4.8 +3.5 years, had good compliance with the special diet; their Phe mean was 192+ll .5/nnol/L. Group B (n=26), mean age 7.9+3.7 years, did not adhere very well to this diet; their Phe mean was 597+ 187/maol/L. Group C (n=21), mean age 6.7+2.3 years, was the control group with Phe mean 109_+24/zmol/L. All PKU patients had been put on the special Phe-restricted diet during their neonatal period. Nine (38%) of the patients in group B suffered from seborrhoeic dermatitis or eczema. Blood (5ml) was withdrawn from each individual for biotinidase assay and liver function tests. Phe was measured with a quantitative enzymatic assay on dried blood samples on filter paper (2992 Schleicher-Schull) with Porton Cambridge reagents. Biotinidase activity was assayed with a fluorimetric method (Ebrahim and Dekisnamur 1986). At the time of blood collection for biotinidase assay, the mean phenylalanine concentration was 133+24/nnol/L in group A, which was statistically significantly different from that of group B at 473+127/zmol/L. Biotinidase activity in group A (5 .58+l .77nmol /min per L) was not significantly different from the controls (5.98+2.08nmol/min per L). However, the mean biotinidase activity in group B (4.21 + 1.68nmol/min per L) was significantly decreased (p--0.006) compared to that of group A as well as to that of controls (p<0.001). Biotinidase activity in group A did not correlate with mean phenylalanine concentration (r=0.08) nor that at the time of investigation (r=0.23). In contrast, in group B biotinidase activity correlated negatively with Phe mean ( r=-0 .59) and Phe concentration at the time of investigation (r---0.60). Vegetables and fruits, which are the main part of the PKU diet (group A), contain watersoluble forms of biotin. The PKU children in group B, who do not adhere well to the special diet, consume more animal products, in which biotin is in firmly bound complexes insoluble in water. However, biotin deficiency does not alter biotinidase activity in vitro: the activities in the sera of several patients who became biotin-deficient were normal (Wolf