A lack of consensus and few data support testosterone replacement therapy (TRT) in hypogonadal men who have been treated for prostate cancer (CaP), particularly those who have received radiation therapy. We performed retrospective review of 13 hypogonadal men with CaP, treated with brachytherapy or external beam radiotherapy who were subsequently treated with testosterone (T) between 2006 and 2011. Serum T, free T (FT), estrogen (E), sex hormone-binding globulin (SHBG), prostate-specific antigen (PSA), hemoglobin (Hgb) and hematocrit (Hct) values were evaluated approximately every 3 months after TRT initiation up to 67 months of follow-up. Prostate biopsies demonstrated four men with Gleason (Gl) 6, 7 with Gl 7 and 2 with Gl 8 disease. Median (interquartile range) age at TRT initiation was 68.0 (62.0-77.0) years, initial T 178.0 (88.0-263.5) ng dl(-1), FT 10.1 (5.7-15.0) pg ml(-1) and PSA 0.30 (0.06-0.95) ng ml(-1). Median follow-up after TRT initiation was 29.7 months (range 2.3-67.3 months). At median follow-up, a significant increase in mean T (368.0 (281.3-591.0) ng dl(-1), P=0.012) and SHBG were observed, with no significant increases in Hgb, Hct, E, FT, or PSA (0.66 (0.16-1.35) ng ml(-1), P=0.345). No significant increases in PSA or CaP recurrences were observed at any follow-up interval. TRT in the setting of CaP after treatment with radiation therapy results in a rise in serum T levels and improvement in hypogonadal symptoms without evidence of CaP recurrence or progression.