Autosomal dominant polycystic kidney disease (ADPKD) is a genetically heterogeneous disorder A mutation in at least three different genes can cause the disease. A mutation in the first gene, the PKD1 gene, which has been identified on chromosome 16p13.3, accounts for ADPKD in approximately 86% of the families with this disorder. In the majority of the other ADPKD families the disease is caused by a mutation in a second gene, the PKD2 gene. This gene has been mapped to chromosome 4q21-22, but has not yet been identified. In a few families ADPKD is not caused by a mutation in either the PKD1 or the PKD2 gene. The locus for a possible third gene has not yet been determined. Now that haplotype analysis with polymorphic markers at the ADPKD1 and ADPKD2 loci is possible, we can easily distinguish between both forms of ADPKD. We describe a large Dutch family in which ADPKD is linked to chromosome 4. Compared with ADPKD1 families, the disease in this family tends to run a milder course, as has been described previously for other ADPKD2 families.