A group of 13 Moroccan patients with MPS I and their families, including three siblings and twin siblings, was screened for mutations of the α-L-iduronidase gene using fluorescence-assisted mismatch analysis (FAMA) and cycle sequencing of PCR products. The P533R mutation, which is rare in Europeans, was identified in 92% of mutant alleles (24/26). This is the highest frequency of this mutation detected in patients with Hurler syndrome. None of the patients carried the W402X or Q70X alleles, the most common MPS I mutations in Europeans. These results suggest that the P533R mutation constitutes the genetic lesion which results in MPS I in people of Moroccan descent and provides yet more evidence for the uneven geographical distribution of mutations in MPS I.