Flow cytofluorometric (FCF) DNA analysis was performed on specimens from 154 patients with non-Hodgkin's lymphoma (NHL) and correlated to histopathology according to the Rappaport classification and the Working Formulation of NHL for clinical usage (WF). NHL associated with unfavorable prognosis (UP) had significantly higher proportions of cells in S-phase as compared to the favorable prognosis (FP) group. The proportion of S-phase cells also differed significantly between the low-, intermediate-, and high-grade malignancy groups of the WF. Aneuploidy was significantly more frequent in the UP (41%) than in the FP (15%) group. By stepwise discriminant analysis, the S-phase frequency was a stronger discriminator between prognostic groups than DNA content. Classification by discriminant analysis showed that over 90% of the low grade/FP lymphomas were allocated to the good prognosis group by their S-phase and DNA values. Due to the variation in the proportion of S-phase cells in the intermediate grade and high grade, as well as the UP lymphomas, only approximately 50% of the UP and 60% of the high-grade lymphomas were identified as such by this discriminant model, while almost one half of the intermediate-grade lymphomas were allocated to the low-grade group. The results suggest that S-phase analysis can be of value in distinguishing intermediate- and high-grade lymphomas with high and low proportions of proliferating cells, which may be prognostic importance.