Acute fatty liver of pregnancy is a disease of the third trimester which is generally considered to be rare and to have a grave prognosis. This study found an optimistic outlook for patients with acute fatty liver of pregnancy due to early termination of the pregnancy as well as the recognition of milder cases. In prospectively followed women, a maternal mortality of 8% and a fetal mortality of 14% were observed. The disorder also appears to be more common than previously suspected and should be considered in all women with liver dysfunction in late pregnancy, even if they are anicteric. Histologically, the characteristic fine droplet steatosis usually produces distinct vacuoli-zation in sections stained with hematoxylin-eosin. However, early in the course of the illness, liver cells have a ballooned appearance and the presence of lipid is masked. When accompanied by a significant necroinflammatory reaction, this stage may be difficult to distinguish from acute viral hepatitis. Whenever acute fatty liver of pregnancy is suspected, a small piece of the biopsy should be reserved for special stains to confirm the presence of lipid in frozen sections. Significant loss of hepatic parenchyma is a regular accompaniment of acute fatty liver of pregnancy and is due to hepatocytolysis, acidophilic degeneration and liver cell atrophy. Extramedullary hematopoiesis and giant mitochondria are often present; the latter change is probably an adaptive or degenerative response to an altered metabolic environment. Despite the frequent presence of signs and symptoms of toxemia in patients with acute fatty liver of pregnancy, no histologic overlap was observed, suggesting that they represent distinct etiologic entities.