Flow cytometry studies for cellular DNA analysis were performed in 115 cases of non-Hodgkin's lymphoma, 53 of which had not received any prior chemotherapy or radiotherapy. DNA content was measured in ethanol fixed cells stained with chromomycin A3. According to the criteria of the International Working Formulation there were 43 low grade, 58 intermediate grade, and eight high grade lymphomas; six cases were in the miscellaneous group. Seventy seven (67%) had only diploid DNA content. Thirty eight (33%) showed DNA aneuploidy; 20 of these had been previously treated with chemotherapy or radiotherapy, or both. DNA aneuploidy was seen as hyperdiploidy in all cases except one, and it varied from slightly hyperdiploid to tetraploid. The incidence of aneuploidy increased significantly with increasing histological grade (p = 0.0002) and was not related to previous treatment. The low, intermediate, and high grade lymphomas had 14% (six of 43), 47% (27 of 58), and 62.5% (five of eight) cases, respectively, that showed DNA aneuploidy. The percentage of cells in S phase increased significantly with a higher histological grade (p less than 0.0001). The median S fraction in the low, intermediate, and high grade lymphomas was 1.0 (0.5 to 10)% 4 (0.4 to 35)%, and 27 (4.6-56)%, respectively. There is a significant correlation between histological grade and S fraction and the presence or absence of aneuploidy. There is heterogeneity, however, within both histological grade and a histological subtype.