Four different gene mutations were identified in five unrelated Portuguese patients with pyrimidine 5'-nucleotidase type I (P5'N-I) deficient chronic hemolytic anemia. Mutations 502G-->C (168Gly-->Arg), 773T-->C (258Ile-->Thr) and the insertion of an Alu element in exon 9, leading to skipping of this exon in the mRNA transcript, are newly described mutations whereas mutation 425T-->C (142Leu-->Pro) has been previously reported.