Hemoglobin (Hb), hematocrit (Hct) and mean corpuscular hemoglobin concentration (MCHC) were determined in 36 acyanotic and 38 cyanotic patients with congenital heart disease. Serum iron (SI) was also determined in 16 cyanotic patients. Patients were classified as follows: Gp I, 0.5-5 years old and Gp II, 6-12 years. In acyanotic patients, anaemia (Hb 11 gr%) was present in 35% for Gp I and 18.7% for Gp II (Hb 11.5 gr%), a figure which is similar to the general Greek children's population. In cyanotic patients which have normal or high Hb concentration, when SI was used as an index, anaemia (SI 60 g%) was found in 37.5% for Gp I and 12.5% for Gp II. Furthermore, when using as index the MCHC the percentage of anaemia (MCHC 31%) was further increased in those patients (44% for Gp I and 23.8% for Gp II). Four cyanotic infants with MCHC 31% developed cerebrovascular accidents. The data show 1) that the MCHC is a convenient and accurate index of iron deficiency anaemia in children with cyanotic heart disease and 2) that the incidence of hypochromic anaemia is high in cyanotic patients.