We studied PTEN/MMAC1, a newly discovered candidate tumor suppressor gene at 10q23.3, for mutations in lung cancer. One hundred and thirty-six lung cancer cell line DNAs (66 small cell lung cancers, SCLC, 61 non-small cell lung cancers, NSCLC, four mesotheliomas, five extrapulmonary small cell cancers) were analysed for PTEN/MMAC1 homozygous deletions and five (8%) SCLC lines showed homozygous deletions interrupting the PTEN/MMAC1 gene. Using single stranded conformation polymorphism (SSCP) analysis, we screened the PTEN/MMAC1 open reading frame of 53 lung cancer cell line cDNAs for point mutations and found that 3/35 SCLCs and 3/18 NSCLCs contained homozygous amino acid sequence altering mutations. Northern blot analysis revealed that expression of the PTEN/MMAC1 gene was considerably lower in all the tumor cell lines with point mutations while no expression was detected for cell lines with PTEN/MMAC1 homozygous deletions. Mutation analysis of 22 uncultured, microdissected, primary SCLC tumors and metastases showed two silent mutations, and two apparent homozygous deletions. We also discovered a processed pseudogene (PTEN2) which has 98.5% nt identity to PTEN/MMAC1, that needs to be accounted for in cDNA mutation analysis. Our findings suggest that genetic abnormalities of the PTEN/MMAC1 gene are only involved in a relatively small subset of lung cancers.