12 cases of hereditary myeloperoxidase (MPO) deficiency are reported. Histochemical stainings, lysosomal enzyme determinations, electron microscopic study of MPO and granulocytic function were performed. Family studies on 2 generations were carried out in 5 patients and histochemical stainings and biochemical lysosomal enzyme determinations were done. MPO deficiency was found to follow autosomal recessive inheritance and only rarely to have clinical effects.