We monitored serum liver tests in 46 obese persons undergoing rapid weight reduction with a supplemented fasting diet containing a mixture of protein, glucose, essential fatty acids, vitamins, and minerals. Levels of SGOT, SGPT, alkaline phosphatase, lactic dehydrogenase (LDH) and serum concentrations of bilirubin, total protein, and albumin were within normal limits at the onset of the study. SGOT, SGPT, LDH, alkaline phosphatase, and serum total bilirubin rose mildly but significantly 2 weeks after the institution of the supplemented fasting program. Thereafter, the values began to decline, approaching baseline levels within 4-6 weeks despite the continued dietary restrictions. We conclude that rapid weight reduction using a supplemented fasting regimen can lead to a mild hepatic test abnormality. The resultant liver test abnormalities are transient and reversible and may not require extensive diagnostic investigations.