The nuclear DNA content of 37 primary non-Hodgkin's lymphomas both at presentation and at relapse was determined by flow cytometric analysis from paraffin-embedded tissue to investigate changes in DNA ploidy and S-phase fraction (SPF) during the course of the disease, and their association with survival. The repeat biopsies were done from 5 months to 15 years after the diagnosis. Four low-grade lymphomas according to the Working Formulation transformed into intermediate-grade lymphomas (four of 11, 36%), and four intermediate-grade lymphomas into high-grade lymphomas during the follow-up (four of 16, 25%), and five of these eight transformed lymphomas were fatal within 18 months after relapse. The SPF correlated strongly with poor prognosis if measured either from the primary biopsy (P = 0.008), the first (P = 0.009), or the latest repeat biopsy (P = 0.006). If SPF was greater than or equal to 6% larger in a repeat biopsy than at presentation prognosis was poor; six of nine such patients died from lymphoma within 11 months from recurrence. An increase of greater than or equal to 6% in the SPF was more common in high-grade (four of nine, 44%) and intermediate-grade (four of 16, 25%) lymphomas than in low-grade lymphomas (one of 11, 9%), and it was occasionally (three of nine) associated with a morphologic change. In a few cases a repeat biopsy was diploid despite DNA aneuploidy at presentation. In conclusion, the study provides evidence that not only may low-grade lymphomas transform into higher grade lymphomas, but high-grade lymphomas may also frequently transform into more malignant forms during the course of the disease. The SPF is useful in monitoring the biological behavior of non-Hodgkin's lymphoma, and it appears to give information not obtained by histologic study alone.