The fetotoxic effects of angiotensin converting enzyme inhibitors when used during the second half of pregnancy are well known. The more recently developed angiotensin II receptor antagonists appear to yield similar fetal abnormalities. We report a premature infant born to a 41-year-old mother with a long history of infertility who had received losartan therapy for hypertension throughout an undetected pregnancy. Ultrasound examination 2 days prior to delivery identified a single fetus at 29 weeks gestation, anhydramnios, and an empty fetal bladder. The neonatal course was complicated by oliguria, hyperkalemia, marked renal dysfunction, respiratory failure, joint contractures, and a large anterior fontanelle with widely separated sutures. Hypotension (mean arterial pressure<25 torr) on day 1 responded to volume expansion, dopamine, and hydrocortisone. Serum creatinine reached a maximum of 2.7 mg/dL on day 6 and decreased to 0.4 by day 56. No formal urinalysis was performed, but the urine was reported to be visually clear throughout the course. Although a renal ultrasound on day 2 was normal, a follow-up study at 7 months revealed bilateral generalized parenchymal echogenicity, consistent with medical renal disease. Since then, weight and length have been at the 5th percentile or less, with apparent renal tubular acidosis necessitating the addition of sodium citrate supplements. This case emphasizes the importance of maintaining a high index of suspicion for potential pregnancy when contemplating the use of a drug of this class, and considering serial testing for pregnancy when using such drugs, even in patients with a longstanding history of infertility.