The aim of this study was to investigate incidental testicular doses during intensity modulated radiation therapy (IMRT) in patients treated with prostate radiotherapy only (PORT) and whole pelvis radiotherapy (WPRT). A total of 34 prostate cancer patients with intermediate and high risk were included in this prospective study. Each patient in the intermediate risk group received a total of 78 Gy in 39 fractions for prostate and seminal vesicles. In patients in the high risk group, 2 Gy daily fraction dose for pelvic lymphatics was given to 50 Gy, and then 78 Gy was given to prostate and seminal vesicles volumes. Treatment plans were created for all patients using the IMRT technique with 6MV. Testicular doses were measured for WPRT and PORT by thermoluminescence dosimetry (TLD) detectors placed on testis surface. Testicular doses measured for WPRT and PORT were compared. The isocenter to testicular distance for WPRT and PORT was 16.83-cm (13.20 to 18.80-cm) and 11.15 cm (9.10 to 13.00-cm), respectively. The mean testicular dose measurements of TPS and TLD per fraction during PORT were 2.41 cGy (1.95 to 3.60 cGy) and 3.70 cGy (2.80 to 5.10 cGy), respectively (p = 0.00). In WPRT irradiation, mean testicular dose values of TPS and TLD per fraction were measured as 3.85 cGy (2.00 to 5.70 cGy) and 5.85 cGy (4.25 to 7.55 cGy), respectively (p = 0.00). The cumulative mean scattered dose for PORT irradiation of 78 Gy in 39 fractions was 144.30 cGy. The mean cumulative dose received by the testis for the high-risk prostate patient was 228.15 cGy. There was a significant difference in testicular dose between WPRT and PORT irradiation. Testicular doses decreased significantly with increasing isocenter-testis distance. Incidental testicular dose during prostate radiotherapy can be significantly detrimental to spermatogenesis. Therefore, the testicles should be contoured as an organ at risk for the estimation of absorbed doses. The use of in vivo dosimetry is recommended for accurate measurement of testicular dose in radiotherapy of prostate cancer for men desiring continued fertility.