To assess the capacity of the corpus luteum to secrete steroid hormones and relaxin during spontaneous abortion, human chorionic gonadotropin (hCG), progesterone, 17 alpha-hydroxyprogesterone (17-OHP) and relaxin concentrations were determined in serum samples obtained from 16 spontaneously aborting first-trimester patients and 5 asymptomatic, apparently normal women who subsequently aborted. hCG and P were subnormal in more than half of aborting patients, and 17-OHP in half of them. However, relaxin was normal in most of them. No patient with subnormal relaxin levels had hCG values in the normal range. Such a relationship was not found between hCG and 17-OHP values. Hence, decreases in placental activity seem to precede a decrease in luteal ability to secrete relaxin but not to secrete steroid hormones. Some aborting patients with subnormal 17-OHP levels had normal relaxin values. All but 1 woman who aborted subsequently had subnormal 17-OHP values at a time when relaxin levels were normal. Therefore, in some aborting patients, the ability of the corpus luteum to secrete steroid hormones is impaired at a time when its capacity to secrete relaxin is preserved.