OBJECTIVE: To examine the DNA content of circulating lymphocytes obtained from HIV-1-infected persons and to explore the effects of antiretroviral therapy on these indices. DESIGN: Cross-sectional analysis and 48-week open label treatment trial (AIDS Clinical Trials Group Protocol 315) of zidovudine, lamivudine and ritonavir. METHODS: Peripheral blood lymphocytes were obtained from HIV-1-infected patients and healthy controls and after 48 h of in vitro cultivation were stained with propidium iodide and analyzed for DNA content by flow cytometry. RESULTS: HIV-1-infected patients had more hypodiploid cells (19%), fewer G0-G1 phase cells (70%) and more S phase cells (10%) than did healthy controls (8%, 85% and 5% respectively; P = 0.002). Patients with sustained suppression of plasma HIV-1 RNA levels after antiretroviral therapy had only modest improvements in these indices. In contrast, patients who failed to suppress plasma HIV-1 RNA levels had decreases in G0-G1 cells to 54% (P = 0.032) and increases in S phase cells to 24% (P = 0.055). Plasma HIV-1 RNA levels and the percentage of S phase cells were correlated (r, 0.23; P = 0.047). In patients failing antiretroviral therapy, there was an inverse correlation between the percentage of G0-G1 cells and expression of the activation antigens CD38 and HLA-DR on CD4 cells (r, -0.409; P = 0.016) and CD8 cells (r, -0.363; P = 0.035). CONCLUSIONS: Lymphocytes obtained from HIV-1-infected patients display perturbations in DNA content after brief cultivation in vitro reflective of immune activation in vivo. The marginal improvement in these indices after 'successful' suppression of HIV-1 replication suggests that even low levels of HIV-1 replication are sufficient to induce immune activation and perturbations in DNA content.