The peroxisome biogenesis disorders (PBDs) are a group of lethal autosomal-recessive diseases caused by defects in peroxisomal matrix protein import, with the concomitant loss of multiple peroxisomal enzyme activities. Ten complementation groups (CGs) have been identified for the PBDs, with CG1 accounting for 51% of all PBD patients. We identified the human orthologue of yeast PEX1, a gene required for peroxisomal matrix protein import. Expression of human PEX1 restored peroxisomal protein import in fibroblasts from 30 CG1 patients, and PEX1 mutations were detected in multiple CG1 probands. A common PEX1 allele, G843D, is present in approximately half of CG1 patients and has a deleterious effect on PEX1 activity. Phenotypic analysis of PEX1-deficient cells revealed severe defects in peroxisomal matrix protein import and destabilization of PEX5, the receptor for the type-1 peroxisomal targetting signal, even though peroxisomes were present in these cells and capable of importing peroxisomal membrane proteins. These data demonstrate an important role for PEX1 in peroxisome biogenesis and suggest that mutations in this gene are the most common cause of the PBDs.