Fludarabine monophosphate (FAMP) has been shown to be highly effective against low-grade malignant B-cell lymphoproliferative diseases. Because some opportunistic infections were observed in patients treated with FAMP, we investigated the influence of this drug on several parameters of immunocompetence. For 17 consecutive patients treated with FAMP for CLL or low-grade malignant lymphoma we studied T-cell subpopulations during and after therapy by flow cytometry and our findings were correlated with the clinical course of their disease. A pronounced decrease in the various T-cell subpopulations was seen in all cases, that for CD4+ cells was still present 11-13 months after the end of the therapy. In 7 patients a severe opportunistic infection developed; the outcome was fatal in 2 cases. Only 5 patients did not experience any serious infection. These results show that FAMP therapy in a dose of 25 mg/m2/day for 5 d every 4 weeks might be too toxic for patients with very advanced disease. However, in view of the efficacy of FAMP, the possibility of less intensive schedules for these advanced cases should be explored.