Niemann-Pick type C (NPC) disease is a rare autosomal-recessive lysosomal storage disease typically accompanied by progressive impairment of nervous system and liver function. Biochemically, the disorder presents with an inhibited egress of cholesterol and glycosphingolipids from endosomal and lysosomal compartments in neuronal and nonneuronal cells. In the majority of NPC patients, mutations in the NPC1 gene can be identified. About 5% of patients show mutations in the NPC2 gene. Many different mutations can cause NPC disease and multiple variants not associated with the disease are known in both genes. A continuously updated collection of gene variants is lacking to date and only limited information is available on genotype-phenotype correlation. We have created the NPC disease gene variation database (NPC-db; http://npc.fzk.de; last accessed 24 October 2007). This database aims to provide a comprehensive overview of the sequence variants in NPC1 and NPC2, including information on their functional consequences and associated haplotypes. Moreover, genotype data and clinical information from individual NPC patients provide information on the impact of functional variants. NPC-db addresses professionals and nonprofessionals dealing with NPC disease on a clinical, diagnostic, research, or personal basis. The user is encouraged to search contents and submit novel information, thereby contributing to generate a valuable open-access tool that will allow a better understanding of the molecular and clinical details of NPC disease.