Small cell lung cancer (SCLC) frequently shows a loss of heterozygosity (LOH) on chromosome 15q. In order to define the commonly affected region on chromosome 15q, we tested 23 primary SCLCs by microsatellite analysis. By analyzing 43 polymorphic microsatellite markers located on chromosome 15q, we found that 14 (60.8%) of 23 tumors exhibited a LOH in at least one of the tested microsatellite markers. Two (14.3%) of the 14 tumors were found to have more than a 50% LOH on chromosome 15q. LOH was observed in five commonly deleted regions on 15q. Of those regions, LOH from D15S1012 to D15S1016 was the most frequent (47.8%). LOH was also observed in more than 20-30% of tumors at four other regions, from D15S1031 to D15S1007, from D15S643 to D15S980, from D15S979 to D15S202, and from D15S652 to D15S642. Four of the 23 tumors exhibited shifted bands in at least one of the tested microsatellite markers. Shifted bands occurred in 3.2% (29 of 914) of the loci tested. Our data suggests the presence of at least five tumor suppressor loci on chromosome 15q in SCLC, and further that these may play an important role in SCLC tumorigenesis.