A patient with acute myeloid leukemia (M4) in the first complete remission received a bone marrow transplantation (BMT) from an HLA-compatible sibling. Sustained engraftment was achieved, but she developed colicky pain at the back and lower quadrant of both sides on days 19-21 post-BMT, followed by hemorrhagic cystitis 13 days later. Ultrasonogram, intravenous pyelogram and computed tomogram of the abdomen showed hydronephrosis and ureteric obstruction of both sides. There was no stone in the urinary tract or abnormality of the bladder. The cortex of the right kidney was rather hypertrophic in spite of the persistent presence of hydronephrosis. Viral culture of urine and electron microscopic examination of urine sediments revealed the presence of adenovirus type II. Infection of the urinary tract with adenovirus type II may have been the underlying cause of the hemorrhagic cystitis and possibly also of the otherwise unexplained ureteric obstruction.