The gene causing adult polycystic kidney disease (APKD) in most northern European families has been localised to the short arm of chromosome 16, close to the alpha globin gene (PKD1). A DNA probe 3' to the alpha globin locus (3'HVR) has been used to test such families but a second genetic locus (PKD2) was recently proposed when two families from Italy failed to show linkage to that locus. The presence of two or more loci could significantly reduce the value of linked probe analysis as a tool for the diagnosis of APKD and we have therefore examined five Mediterranean families using the 3'HVR probe. In these families we have not demonstrated any mutations at the second locus. The 3'HVR gene remains the most useful probe in making the diagnosis of APKD disease in an Australian Caucasian population, and the certainty with which the diagnosis is made may be enhanced by the additional use of other nearby probes when the defect involves the PKD1 locus.