Blood samples from 722 unrelated patients with anemia and/or reticulocytosis were submitted to our laboratory for red cell enzyme assay during the past 7 years. Among these 722 cases, we found 82 cases of 7 different red cell enzyme deficiencies and 2 of unstable hemoglobin. Abnormalities of pyruvate kinase (PK) were found to cause hemolysis in 55 patients. Although their average PK activity was about 35% of the normal level, 5 showed normal and 2 demonstrated high PK activity. Among 17 patients in whom pyruvate kinase assays or screening tests had been carried out in routine laboratories, the correct diagnoses had been made in only 4. Glucose-6-phosphate dehydrogenase (G6PD) deficiency was found in 15 patients, pyrimidine 5'-nucleotidase deficiency in 5, glucose phosphate isomerase deficiency in 3, adenylate kinase deficiency in 2, phosphoglycerate kinase deficiency in 1, and glutathione synthetase deficiency in 1 patient. Even after we performed a panel of over 20 different red cell enzyme assays, 519 patients still remained undiagnosed.