OBJECTIVE To test the hypothesis that, before treatment, prostate cancer patients who demonstrate a high yield of ex vivo radiation-induced micronucleus (MN) in G(0) lymphocytes represent a patient population with a greater-than-average risk of developing radiotherapy (RT)-related morbidity. METHODS We prospectively conducted the cytokinesis-block MN assay of peripheral blood lymphocytes (PBLs) in 38 prostate cancer patients. Before the initiation of RT, PBLs from each patient were irradiated (1-4 Gy). The mean patient age +/- SEM was 68.7 +/- 1.0 years. The clinical stage was T1 in 17, T2 in 15, and T3 in 6. The preoperative prostate-specific antigen level was < or =4 ng/mL in 5, 4-10 ng/mL in 18, and >10 ng/mL in 15. All patients underwent standardized pelvic external beam radiotherapy (range 41.4-50.4 Gy) and boost (range 16-26 Gy). The mean follow-up +/- SEM was 32.8 +/- 4.6 months. At the end of follow-up, a radiation oncologist scored the GI or GU morbidity according to the Radiation Therapy Oncology Group criteria without knowledge of the MN data. RESULTS We found that between the average reactors (n = 25; i.e., patients who had Grade 1 or less RT-related morbidity) and over reactors (n = 13; i.e., patients who developed Grade 2 or greater RT-related morbidity), the differences in the ex vivo radiation dose-response relationship of MN yield in PBLs were highly significant, especially at doses of > or =2 Gy. Also, the development of RT-related morbidity correlated with the radiation dose-response relationship of MN yield in PBLs before treatment, but did not correlate with any of the patients' clinical variables. CONCLUSIONS Our findings suggest that the pre-RT ex vivo radiation dose-response relationship of MN yield in PBLs may be a significant predictive factor for the development of GI or GU morbidity in prostate cancer patients after pelvic RT.