Forty-four polycystic ovary syndrome (PCO) patients were treated for a total of 61 cycles with intermediate-dose pure follicle stimulating hormone (FSH). Patient selection was based on hyperandrogenism, oligoovulation and physical signs. Patients with multiple-factor infertility were excluded from the study. Seventeen conception cycles occurred in 17 patients (pregnancy cycles). The spontaneous abortion rate was 29.4%. Forty cycles did not result in conception (Nonpregnancy cycles, 23 patients). Treatment was discontinued in four patients who had suboptimal response. Sixteen pregnancies (94%) occurred within the first two treatment cycles. Pregnancy and nonpregnancy cycles were compared for characteristics associated with a successful outcome. The data suggest that (1) an intermediate-dose pure FSH protocol is most likely to be successful among more "classic" PCO patients, those with obesity, high body surface area, elevated luteinizing hormone/FSH ratio and higher testosterone; (2) if pregnancy is to occur, it is most likely to within two treatment cycles; and (3) ovarian hyperstimulation is more likely to occur in nonconception cycles.