An adult patient is described with hepatomegaly and sea-blue histiocytes in the bone marrow. A diagnosis of cholesterol ester storage disease was established following enzyme and lipid analyses on liver biopsy and cultured skin fibroblasts. Acid esterase activity was deficient (approx. 5% of controls) in liver and fibroblasts using [14C]-triolein or 4-methylumbelliferyl palmitate as substrates. Cholesterol ester levels were raised about 70-fold in liver, whereas triglyceride levels were only marginally raised. Marked accumulation of cholesterol esters was also demonstrated in cultured fibroblasts. Clinically, the patient responded favourably to phenobarbitone treatment. However, this was not reflected in liver acid esterase or lipid levels.