Gaucher disease is the most common sphingolipid storage disease but genotype only broadly predicts phenotype. The 1604G-->A (1604A;R496H) mutation has been described as having a low incidence among Ashkenazi Jews. The purpose of this study was to ascertain phenotypic expression and prevalence of this mutation among patients with Gaucher disease and among healthy Ashkenazi Jews. Patients in two Gaucher clinics (in the United States and Israel) and from an international Gaucher registry were assessed for frequency and phenotype expression; 200 healthy Ashkenazi Jews were screened as well. Molecular analysis was performed by standard methods. In the Gaucher clinic with mostly Jewish patients, the gene frequency was 1.68% compared with 0.38% in the international registry with mostly non-Jewish patients. Among Ashkenazi Jewish controls, no alleles with 1604A were identified. There was a marked overrepresentation of severe alleles in patients carrying the 1604A mutation, suggesting that many patients who are compound heterozygotes for 1604A are not diagnosed as having Gaucher disease because their disease is presumably so mild as to evade detection. In view of its rarity and mild expression, the inclusion of the 1604A mutation in the standard kit for screening for Gaucher disease is unnecessary.