Common regions of loss of heterozygosity on chromosomes 3, 13, and 17 were determined by restriction fragment length polymorphism analysis in 34 tumors and nine cell lines from 27 patients with small cell lung carcinoma. The common regions of loss of heterozygosity on chromosomes 3, 13, and 17 reside between D3S2 (3p14-p21) and ERBA beta (3p22-p24.1), between D13S1 (13q12) and D13S2 (13q22), and distal to MYH2 (17p13.1), respectively. Allele loss in each of these regions has been previously shown in several human tumors. Thus, the present findings indicate the pleiotropy of recessive genetic lesions in these genomic areas. Cytogenetic analysis was performed on three small cell lung carcinoma cell lines which had allele loss on all three chromosomes, and although chromosome 3p deletions were observed in two of three cell lines, no obvious structural abnormalities involving chromosomes 13 and 17 were detected. Mitotic recombination or mitotic nondisjunction rather than deletion may thus be the frequent chromosomal mechanism for attaining homozygosity of chromosomes 13 and 17 in small cell lung carcinoma.