Flow-cytometric DNA analysis of ploidy and proliferation was implemented on small-cell anaplastic carcinoma of the lung. Metastases (38) in 30 consecutive patients with small-cell anaplastic carcinoma of the lung were examined with 273 fine-needle aspirations. The ratio of the peak channel numbers for the G1 phase of the tumor cells to that of the diploid standard (DNA index) was calculated and used for ploidy identification. Twenty-nine patients were evaluable with respect to DNA index determination. The coefficient of variation of the DNA index determinations was 0.039. In 23 (79%) patients, only 1 cell line was detected. Evidence of the presence of 2 tumor cell clones with different ploidy was obtained in the remaining 6 (21%) patients. Of the 35 malignant clones thus demonstrated, 26 (74%) were significantly different from diploid (P .ltoreq. 0.01). Four (11%) were hypodiploid, 3 (9%) were hypotriploid and 19 (54%) were hypo- or near-tetraploid. Clonal heterogeneity in the tumors of 21% of the patients is a conservative estimate; the true occurrence of clonal heterogeneity in small-cell carcinoma of the lung may be much higher.