To investigate the prognostic implications of DNA flow cytometry in human lung tumors, we analyzed specimens from patients with neoplastic and non-neoplastic lung disease. Most non-neoplastic and normal (taken at the resection border) lung samples yielded a single cell population with diploid DNA content (only two normal lung specimens from two cancer patients had aneuploid DNA content). At least one aneuploid cell subpopulation was seen in 91 percent of NSCLC and 50 percent on SCLC. To show intratumor heterogeneity, multiple-site sampling was done whenever possible in both primary tumor and metastatic sites, revealing a high incidence of multiclonality (50 percent). Although diploid tumors were rare, they associated with a higher survival rate than aneuploid monoclonal and multiclonal tumors with hypoploid and/or hypertetraploid clones, which had the lowest survival. Cellular DNA content analysis in patients with lung tumors may be useful in prognostic evaluation.