Evaluation of two patients with transfusion dependent anemia revealed RBC pyruvate kinase to be 33% and 41% of the mean normal value, with normal or high values of other RBC enzymes. Parental PK activities were just below normal in three of four of the parents. Subsequent DNA analysis revealed both patients to be compound heterozygotes for PKLR gene mutations, two of which are previously undescribed. Borderline low pyruvate kinase activities with increased in other RBC enzyme activities should prompt consideration of measurement of parental enzyme activities, and confirmation by DNA analysis if available.