Clinically reversible veno-occlusive disease of the liver developed in a 23-year-old man with acute lymphocytic leukemia after 10 months of maintenance therapy with 6-thioguanine. Serial liver biopsies showed the development and resolution of intense sinusoidal engorgement. Although this disease was clinically reversible, some subintimal fibrosis about the terminal hepatic veins persisted. This case presented a unique opportunity to observe the histologic features of clinically reversible hepatic veno-occlusive disease over time, and may be the first case of veno-occlusive related solely to 6-thioguanine.