Most cases of veno-occlusive disease/sinusoidal obstruction syndrome (VOD/SOS) occur <21 days after allogeneic hematopoietic stem cell transplant (HCT). Rarely, however, VOD/SOS can occur later, and can be confused with other causes. We report the clinicopathologic features of eight patients with advanced hematologic malignancies developing VOD/SOS >30 days after dose-escalated busulfan/fludarabine/alemtuzumab and HCT. Median time to diagnosis was 52 days (range: 33-77). For seven patients, VOD/SOS was confirmed by liver biopsies showing classical features including reticulin deposition within sinusoids, central vein occlusions, hepatocyte atrophy/necrosis, sinusoidal/perivenular hemorrhage and sparing of portal tracts. VOD/SOS risk was directly related to higher busulfan plasma exposures. Two patients died from VOD/SOS, and in another two patients VOD/SOS was contributory to death. Late-onset VOD/SOS may be underrecognized and should be considered in the differential diagnosis of patients undergoing HCT, particularly after high dose busulfan. Liver biopsy should be entertained even late in the course if appropriate signs/symptoms exist.