The gene resposible for Sanfilippo syndrome type A, a lysosomal disorder caused by deficiency of sulfamidase, was recently cloned and more than 40 mutations were identified. This paper presents the mutation analysis and clinical findings in 11 Spanish patients in whom 19 of the 22 mutant alleles have been identified. This is the first report on mutations in Spanish Sanfilippo A patients. Seven different mutations were found, four of which (Q85R, R206P, A354P, and L386R) were not previously described. Mutation 1091delC was the most prevalent, accounting for nearly one-half of the mutated alleles, while mutations R245H and R74C were not found. Haplotype analysis suggests a founder effect as the cause of the high frequency of 1091delC in this population.