Aim: To evaluate the relationship between testicular function and testicular volume measured by using Prader orchidometry and ultrasonography (US) to determine the critical testicular volume indicating normal testicular func-tion by each method. Methods: Total testicular volume (right plus left testicular volume) was measured in 794 testes in 397 men with infertility (mean age, 35.6 years) using a Prader orchidometer and also by ultrasonography.Ultrasonographic testicular volumes were calculated as length×width×height×0.71. To evaluate volume-function relationships, patients were divided into 10 groups representing 5-mL increments of total testicular volume by each method from below 10 mL to 50 mL or more. Results: Mean total testicular volume based on Prader orchidometry and US were 36.8 mL and 26.3 mL, respectively. Semen volume, sperm density, total sperm count, total motile sperm count, and serum FSH, LH, and testosterone all correlated significantly with total testicular volume measured by either method. Mean sperm density was in the oligozoospermic range in patients with total testicular volume below 35 mL by orchidometry or below 20 mL by ultrasonography. Mean total sperm count was subnormal in patients with total testicular volume below 30 mL by orchidometry or under 20 mL by ultrasonography. Conclusion: Testicular volume measured by either ultrasonography or Prader orchidometry correlated significantly with testicular function.However, critical total testicular volume indicating normal or nearly normal testicular function was 30 mL to 35 mL using Prader orchidometer and 20 mL using ultrasonography. Prader orchidometry morphometrically and function-ally overestimated the testicular volume in comparison to US.