Erythrocyte enzymopathies are being detected with increasing frequency. The homozygous state for pyruvate kinase (PK) deficiency usually causes chronic non-spherocytic haemolytic anaemia. Aggravation of the condition by a variety of extrinsic factors has been reported, but drugs have rarely been incriminated. A case of severe PK deficiency is presented in which an anovulant drug upsets the usual haematologic balance. This strengthens the suspicion that contraceptive pills may be hazardous in such patients.