OBJECTIVE: To determine the accuracy of the Prader orchidometer and ultrasound (different methods) for measuring the testicular volume by comparing the resultant measurement with the actual testicular volume in humans. BACKGROUND: Since seminiferous tubules comprise 80 to 90% of testicular mass, the testicular volume is believed to be an index of spermatogenesis. Therefore accurate testicular volume is one way to assess testicular function. MATERIALS AND METHODS: The testicular volumes of 121 testes from 62 patients with prostate cancer (mean age 72.74+ 9.38) were measured using the Prader orchidometer and ultrasound before therapeutic bilateral orchidectomy. The ultrasound measurements of testicular volume were calculated using three formulas: length (L) x width (W) x height (H) x 0.52, L X W2 X 0.52, L X W X H X 0.71. The actual testicular volumes were determined by water displacement of the testis. RESULTS: The mean actual testicular volume of the 121 testes was 10.6+3.5ml. A strong correlation was found between the actual testicular volume and volume calculated by the three ultrasound formulas (r=0.853 to 0.871, p= 0.0001) and Prader orchidometer (r=0.921,p=0.0001). The smallest mean difference from the actual testicular volume was observed with the formula L X W X H X 0.71, which underestimated the actual volume by 0.41ml (3.89%). The measurements using the Prader orchidometer correlated with the testicular volume calculated using the three ultrasound formulas (r=0.848 to 0.854, p= 0.0001). However the orchidometer measurements had a mean difference from the actual testicular volume of 2.66ml (25.14%) CONCLUSION: The result of this study has shown that measuring the testicular volume by ultrasonography is more accurate than by Prader orchidometer, and the formula L X W X H X 0.71 is the most accurate for calculating the testicular volume.