Autosomal dominant polycystic kidney disease (ADPKD) is one of the commonest genetic diseases in man, affecting 1:1000 individuals in the Caucasian population. It is caused by mutations in the PKD1 or PKD2 genes. Recently, controversial data regarding the mutational mechanism underlying cyst initiation have been reported: genetic analyses have shown that second somatic mutations may lead to cyst formation (detected as microsatellite loss of heterozygosity, LOH, and point mutations), but immunohistochemical studies show strong immunoreactivity for polycystin in some cysts. In order to further characterise this matter we have analysed 211 cysts from seven different patients for LOH, we have detected a 13.3% LOH for PKD1. This loss was specific to PKD1 as no LOH was detected when other chromosomal regions were studied. Whenever linkage analysis has been possible, it has been proved that the lost allele corresponded to the wild-type. Our data supports previous results in the two-hit theory for ADPKD due to the large number of cysts studied. ADPKD would occur through a recessive cellular mechanism. The probability of cyst development would depend on the probability of mutation in the second allele. The different phenotypical expression of the same mutation reported in ADPKD could be due to the different tendency of inactivation in the second allele in each individual.