A 15-year-old boy was admitted to our hospital because of microcytic hypochromic erythrocytosis and hyperbilirubinemia in October 1996. The laboratory findings were RBC: 597 x 10(4)/microliter, Hb: 13.1 g/dl, Ht: 40.8%, MCV: 70fl, MCH: 22pg, total bilirubin: 3.2 mg/dl (indirect: 2.2 mg/dl), s-Fe: 99 micrograms/dl, and ferritin: 25 ng/ml. Routine liver function tests were normal. There were no findings of hemolysis except for an increase in serum indirect bilirubin and reticulocytes. Decreased erythrocyte osmotic fragility was observed. The patient's mother and sister also showed microcytic hypochromic erythrocytosis. PCR analysis of genomic DNA from this patient, his mother, and his sister confirmed the diagnosis of the alpha-thalassemia trait. However, the bilirubin-UDP-glucuronosyltransferase 1 (B-UGT 1) gene mutation and the findings of the fasting test indicated the simultaneous presence of Gilbert's syndrome. The association of these two diseases in the same patient appears to be rare, especially in Japan because of the low incidence of thalassemia in this country. We concluded that the hyperbilirubinemia was caused by decreased bilirubin clearance, not by increased erythrocyte destruction.