We performed prospective hormonal studies in 9 patients (5 active and 4 inactive) with systemic lupus erythematosus (SLE) during pregnancy (Weeks 10 to 37). Nine healthy pregnant women and 5 patients with rheumatoid arthritis (RA) were used for comparison. Serum prolactin (PRL), testosterone and estradiol (E2) levels were determined by RIA. The patients with SLE showed higher serum PRL levels, the difference being statistically significant at Week 20, and reaching the highest levels at Weeks 30 to 40 (p = 0.05 when compared to healthy pregnant women). The 5 patients with active SLE had the highest serum PRL levels; one of these had fetal wastage. In active SLE the serum testosterone and E2 levels were decreased significantly from Weeks 10 to 30 compared with controls (p = 0.001). In patients with RA serum PRL levels, although higher than in controls, did not differ significantly, nor did the lower testosterone and E2 levels. We conclude that gonadal hormones and PRL changes observed in SLE are present also during pregnancy and may be related to fetal wastage and reactivation of disease.