In order to identify CLL patients for whom irradiation could be beneficial, we investigated the relationship between in vitro radio-induced apoptosis of leukemic cells and response to low-dose splenic or lymph node radiotherapy. Fourteen patients were included in the in vitro study. Leukemic cells were analyzed by Hoechst staining immediately after collection or 24h of culture following in vitro irradiation of 0-10 Gy. The tumor response rate was 47% (one CR and six PR), with a mean duration of response of 3 months (range: 1-4). A high correlation between tumor response and in vitro tests was observed (p < 0.01) and the positive predictive value of the in vitro tests for tumor and hematological responses was 100% at 5 Gy. These results suggest that the sensitivity of leukemic cells to irradiation should be first evaluated in an in vitro assay to spare refractory patients from the useless toxicity radiotherapy.