Analysis of 49 kindreds with the familial form of medullary thyroid carcinoma (MTC) reported up to 1973 revealed a non-random sex distribution of 220 affected members, with a predominance of females among patients presenting with MTC and of males among patients presenting only with phaeochromocytoma. This pattern of sex distribution has completely disappeared among 201 patients from 47 kindreds reported in more recent years, MTC now being equally distributed between the two sexes. It is suggested that this change is related to the different diagnostic approach predominant in each period of time and that MTCs discovered at a preclinical stage by calcitonin screening are equally distributed between the sexes, while females predominate when tumours have progressed until they are clinically evident.