The clinical course of 123 pregnancies in 86 patients with biopsy-proven glomerular diseases have been studied. In 35 women the onset of nephropathy occurred during pregnancy. No complications were observed in more than half of the pregnancies. In the others, one third of the complications were obstetrical or fetal accidents, one third were renal manifestations (hypertension or deterioration of renal function) and one third were both causes. The lowest incidence of complications was observed in patients with membranous nephropathy and the highest in membranoproliferative glomerulonephritis patients. There were 6 spontaneous late abortion, 6 stillbirths and 5 neonatal deaths. 17 deliveries were preterm and 7 fetuses were small for gestational age. Hypertension appeared in 24 pregnancies, in 13 of which it was reversible and related to superimposed preeclampsia and in 11 it persisted after delivery (5 of these 11 pregnancies were in patients with IgA nephropathy). Renal function deteriorated in 10 cases during pregnancy. The deterioration was reversible in 6 and progressive in 4 (2 of whom had membranoproliferative glomerulonephritis). It is suggested that in most patients pregnancy does not change the natural history of glomerular disease.