Among 828 patients undergoing IVF-embryo transfer treatment, the implantation and pregnancy rates of patients who developed < or = 3 follicles were compared prospectively with those patients who had a normal response. Patients who developed 1 to 3 follicles during ovarian stimulation elected to proceed with oocyte collection, have intrauterine insemination if appropriate, or to have their cycle cancelled. In the group of patients who developed < or = 3 follicles and who were aged <40 years, despite a significantly lower number of oocytes collected [2 versus 7; median difference (MD) = 9; confidence interval (CI) = 7-11, and lower number of embryos developed and transferred (1 versus 3; MD = 2; CI = 1-2), no difference in either implantation rate [27.8 versus 20.4%; odds ratio (OR) = 1.58; CI = 0.46-4.54] or pregnancy rate (27.8 versus 36.7%; OR = 0.7; CI = 0.2-2.0) was noted when compared with similarly aged patients who developed >3 follicles. However, in patients aged >40 years who developed < or = 3 follicles, a moderate, albeit non-significant decrease in implantation rate (3.8 versus 7.8%; OR = 1.91; CI = 0.4-57.0) and pregnancy rate (4.2 versus 18.3%; OR = 1.92; CI = 0.38-57.0) was observed when compared with patients of a similar age who developed >3 follicles. Patients aged <40 years, unlike older patients, maintain good implantation and pregnancy rates despite a poor response to ovarian stimulation. This study indicates that for this group of women, continuation of IVF treatment is a better option than cancellation.