The frequency of polycystic ovarian disease (PCOD) as a cause of oligo-amenorrhea and infertility was determined, first by characterizing clinically occult PCOD using endocrinological methods, and secondly by estimating the frequency of overt and occult PCOD amongst infertile women residing in a particular area. Four groups of infertile women with oligo-amenorrhea due to 'functional' disorder were compared. The results show that by contrast with the groups having hyperprolactinemia or hypothalamic disorder the group with hirsutism (and therefore presumed PCOD) was closely resembled by a non-hirsute group in terms of estrogenization, LH level, LH/FSH ratio, prolactin level, body mass and responsiveness to clomiphene. The last group was therefore concluded to have a mild occult form of PCOD. The population studies revealed, first, that overt and occult PCOD accounted for 90% of patients with oligomenorrhea and 37% with amenorrhea, or 73% with oligo- or amenorrhea. Oligo- or amenorrhea accounted for 21% of couples with infertility and the annual incidence was 247 patients per million of the general population. The annual incidence of infertility due to PCOD per million was 41 with overt PCOD and 139 with occult PCOD (total 180). Of those, 140 appeared to respond well to clomiphene (78%) but 40 (22%) failed, requiring alternative therapy.