Among the patients enrolled in the Norfolk In Vitro Fertilization Program there were 32 who had been stimulated according to the basic stimulation protocol using two ampules of human menopausal gonadotropin (hMG) daily. Because of their inadequate response, 23 of these 32 patients were stimulated subsequently with a combination of two ampules of "pure" follicle-stimulating hormone (FSH) and two ampules of hMG on cycle days 3 and 4. The remaining nine patients received four ampules of "pure" FSH only on cycle days 3 and 4. Stimulation was continued with hMG in both FSH regimens. Ten thousand units of human chorionic gonadotropin was used for final maturation. Parallel with the increase in the ratio of exogenous FSH to luteinizing hormone, an increase in oocyte recovery was observed, as well as an improvement in transfer and pregnancy rates. It was concluded that FSH enrichment had a beneficial effect in these patients.