The polymerase chain reaction was used to detect four mutations in the DNA of 47 unrelated patients with type I Gaucher's disease (94 Gaucher's disease alleles). Two of the mutations, 1226 and 1448, and a new mutation (XOVR) representing cross-over between the glucocerebrosidase gene and its closely linked pseudogene, were found. There were five genotypes--namely, 1226/1226, 1226/1448, 1226/XOVR, 1226/?, and ?/? (where "?" indicates that none of the four known mutations was present). Severity of the disease was assessed with a scoring index according to age at diagnosis and extent of organ involvement. Mutation 1226 was associated with a mild clinical phenotype, and mutation 1448 with a more severe phenotype. Mutation 1226 is the most common cause of Gaucher's disease in Jewish patients.