The ovarian hyperstimulation syndrome (OHSS) has become more and more frequent in relation to the spread of the method of superovulation by means of pharmacologic induction and consequently the necessity of measures suitable for preventing and controlling it has been more and more improved. Among them the discovery of indicators which could reveal as early and sensitive as possible the onset and, when it is possible, also the degree of severity of this syndrome, holds an important position. Till now only the serum estradiol concentration performed this task, among the other things, in an incomplete and not-well defined manner, but its inadequacy has stimulated once again the search for new substances which would allow attention to be drawn to the development, entity and evolution of this syndrome in compliance with necessary requisites with greater timeliness, and tenacity. Therefore the behaviour of the serum CA-125 levels has been evaluated for the first time in 31 cycles of 31 patients, which were stimulated by gonadotropins (FSH, FSH/LH, HCG), of which 21 underwent ovarian hyperstimulation of various degree (10 of I, 8 of II and 3 of III grade), while the other 10 had no complications. The blood levels of this marker rose above the limit considered physiological (35 UI/ml) during the luteal phase in all the stimulated cycles and, in hyperstimulated ones always exceeded 70 UI/ml; in addition the serum CA-125 levels correlated very well with ovarian dimensions, degree of severity of the syndrome and estrogenic concentrations during only luteal phase, in the hyperstimulated cycles and continued to remain high or even increased in case of pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)