To determine outcomes in pregnancies complicated by class F diabetes mellitus cared for at a single center, and to assess renal function in these women. A retrospective review (1988-1994) of all pregnant women with class F diabetes was performed, as well as an evaluation of current renal function. Forty-six pregnancies in 45 women reached a mean (+/- standard deviation) gestational age of 35.8 +/- 2.3 weeks, with a mean birth weight of 2623 +/- 818 g. No deliveries occurred before 30 weeks and 39 (84%) were at least 34 weeks. Perinatal survival was 100%. Women with initial serum creatinine exceeding 1.5 mg/dL or more than 3 g/24 hours proteinuria had an increased risk of early delivery, lower birth weight, preeclampsia, and cesarean delivery. Twenty-four individuals (53%) developed preeclampsia and seven met criteria for severe preeclampsia. By the third trimester, 26 women(58%) had greater than a 1 g/24 hour increase in proteinuria and 16 (36%) demonstrated more than a 15% fall in creatinine clearance. Follow-up was obtained in 34 subjects with a mean duration of 2.8 years. Individuals with initial creatinine clearance greater than 90 mL/minute and less than 1 g of protein per 24 hours had less loss of renal function at follow-up, as measured by creatinine clearance. At follow-up, mean protein excretion had decreased 1.9 g/24 hours from third-trimester values, but eight women (24%) maintained protein excretion exceeding 3 g/24 hours. Modern management of class F patients can result in good perinatal outcomes. Renal function studies early in pregnancy can be used to define the risk of perinatal morbidity and long-term progression of renal disease.