A cohort study of adverse reproductive outcome was conducted on a large sheep-raising enterprise in Peru. A total of 673 adult ewes were followed through one reproductive cycle. Assessment endpoints included normal live birth, stillbirth, infertility and perinataldeath within the first 72 h of life. An abnormal reproductive outcome was experienced by 41.5% of ewes. The incidence rates for stillbirth, infertility and perinatal death were 3.0, 32.1 and 6.4%, respectively. Blood samples were obtained from all ewes at the onset of the breeding season and again at lambing. A total of 211 sera from ewes with adverse reproductive outcome were tested for antibody to Toxoplasma gondii. Approximately 27% of ewes were seropositive at the time of initial sampling. Over 30% of ewes seroconverted during the 4 months of observation. Among seroconverters, the frequency distribution of stillbirth, infertility and perinatal death was similar to that observed among ewes which remained seronegative. Among ewes which were seropositive at time of initial sampling and remained seropositive, only infertility was observed. The results suggest that agents other than Toxoplasma gondii may be playing a role in the occurence of stillbirth, infertility and perinatal death in this population. © 1989.