Flow-cytometric DNA analysis yields information on ploidy and proliferative characteristics of a cell population. The analysis was implemented on small-cell anaplastic carcinoma of the lung using a rapid detergent technique for the preparation of fine-needle aspirates for DNA determination and a newly developed procedure for storing aspirates at -80 degrees. Thirty-eight different metastases in 30 consecutive patients with small-cell anaplastic carcinoma of the lung were examined with a total of 273 fine-needle aspirations. The results on ploidy are reported in this paper. The degree of contamination of the aspirates with normal cells was determined by differential counts. 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 estimated as 0.039. In 23 (79%) patients, only one cell line could be 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 less than or equal to 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. Assessment of the detection limit set by the methodology used and the restricted number of samples studied in each patient indicate that the true occurrence of clonal heterogeneity in small-cell carcinoma of the lung may be much higher.