The course of 66 pregnancies was studied in 48 women with primary glomerular diseases. In all cases diagnoses were established by biopsy before pregnancy. They were: membranoproliferative glomerulonephritis in 16 patients, focal glomeruloesclerosis in 13, IgA nephropathy in 10, membranous nephropathy in seven and focal glomerulonephritis in two women. The clinical status of the nephropathy before conception was that 43 had only mild renal dysfunction, five had moderate renal insufficiency, serum creatinine (1.3 to 1.9 mg%), eight women had hypertension (150/100 mm Hg) and eight had nephrotic range proteinuria. Their clinical course was compared with a control group of 36 women with primary glomerular disease who did not become pregnant, and were matched for similar age, histological type, and status of nephropathy (renal function, blood pressure and proteinuria). After one year and at the end of the five year follow-up period, the incidence of hypertension, proteinura, and renal failure was similar in the two groups. The fetal survival rate was 92%; 51 pregnancies ended in full-term delivery, with a mean birthweight of 3,242 ± 320 g. There were seven pre-term deliveries (2,170 ± 135 g), three small for gestational-age (2,340 ± 135 g), two stillbirths and three spontaneous abortions. Three patients had more pre-term deliveries (10.6%) and perinatal mortality (31‰) than a normal population (5.5% and 9.6‰, respectively). Blood pressure increased during pregnancy in 13 women; in 10 it was reversible, and in four it persisted after delivery. Ten gravidas developed increased proteinura (reversible in six of them) and two others developed permanent impairment of renal function. Women with membranoproliferative glomerulonephritis appeared to fare worse, and those with IgA nephropathy and membranous nephropathy better than the rest. Our results suggest that pregnancy does not change the natural course of primary glomerular disease adversely.