Use of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) continues in about 25 countries. This use has been justified partly by the belief that it has no adverse consequences on human health. Evidence has been increasing, however, for adverse reproductive effects of DDT, but additional data are needed. Pregnant women who enrolled in the Collaborative Perinatal Project (United States, 1959-1965) were asked about their previous pregnancy history; blood samples were drawn and the serum frozen. In 1997-1999, the sera of 1717 of these women who had previous pregnancies were analyzed for 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE), the major breakdown product of DDT. The odds of previous fetal loss was examined in relation to DDE level in logistic regression models. Compared with women whose DDE level was <15 microg/L, the adjusted odds ratios of fetal loss according to category of DDE were as follows: 15-29 microg/L, 1.1; 30-44 microg/L, 1.4; 45-59 microg/L, 1.6; and 60+ microg/L, 1.2. The adjusted odds ratio per 60 microg/L increase was 1.4 (95% confidence interval 1.1-1.6). The results were consistent with an adverse effect of DDE on fetal loss, but were inconclusive owing to the possibility that previous pregnancies ending in fetal loss decreased serum DDE levels less than did those carried to term.