Seventy lung tumors from 53 patients were analysed for alterations of myc family oncogenes, c-myc, N-myc and L-myc, to evaluate when activation of these genes occurs during tumor development. The 53 cases were 17 small cell carcinomas (SCCs), 18 adenocarcinomas, 12 squamous cell carcinomas (SqCs), 4 large cell carcinomas and 2 adenosquamous carcinomas. Either N-myc or L-myc was amplified in 4 of the 17 (one N-myc and 3 L-myc) SCCs (24%), while c-myc was amplified in 3 of the 12 SqCs (25%). In one SCC, amplification of N-myc was found in the primary tumor, a pulmonary hilar lymph node metastasis and a pleural metastasis, but not in a liver metastasis or a para-aortic lymph node metastasis. In one SqC, c-myc was amplified in a pleural metastasis and a lymph node metastasis, but not in the primary tumor. In 2 cases of SCCs, amplification or rearrangement of c-myc was detected only in the cell lines, but not in the original tumors taken from the same individuals. These results indicate that tumor cells were heterogeneous for amplification and rearrangement of myc family oncogenes, and suggest that activation of these oncogenes in SCCs and SqCs occurs not at the time of malignant transformation but during tumor progression.