Of 170 patients having undergone orchiopexy for unilateral cryptorchid testes, 90 responded to our inquiry. Testicular size was measured in 55 patients using an accurate and practical orchiometer (punched-out orchiometer) developed in our department. In boys under ten years of age (prepubertal), testicular development was minimal, and the operated on and contralateral testes were not significantly different. With the onset of puberty, both testes enlarged, and no significant differences were observed. However, by age fourteen the side operated on was significantly (p less than 0.05) less well developed than the contralateral testis. No difference was detected clinically between the operated on and contralateral testes when the contralateral testis was less than 10 mL in volume. When the contralateral testis was greater than 15 mL, however, a significant difference was detected both statistically (p less than 0.05) and clinically. The findings suggest limited prospects for the development of testes that are operated on at puberty.