Glucose-6-phosphate dehydrogenase (G6PD) screening and Heinz body preparation are both negative, and the haptoglobin level is decreased at 2.2 mg/dl (normal range: 27–139 mg/dl). C3 and C4 complement levels are normal, and the antinuclear antibody titer, anti-streptolysin O titer and Coombs tests are all negative. The paucity of intact red blood cells and the presence of pigment casts in the urinary sediment in this dark reddish-brown urine suggest the presence of hemoglobin or myoglobin. The moderate anemia, elevated reticulocyte count, and decreased haptoglobin are consistent with intravascular hemolysis. There are autoimmune causes of hemolytic anemia and G6PD deficiency can cause hemolytic anemia, but the Coombs test and G6PD preparation are normal. The fact that more than one hematopoietic cell line is abnormal suggests a clonal disorder. The history of mild leukopenia, thrombocytopenia, and hemolysis is consistent with paroxysmal nocturnal hemoglobinuria. The presence of a cell population lacking decay-accelerating factor (DAF) on the cell membrane determined by flow cytometric analysis with anti-DAF fluorescent antibody confirms the diagnosis.