Testicular volume was assessed in 1029 infertile men and found to be normal in 704; 71 patients had unilateral and 213 had bilateral testicular hypotrophy; 4 had unilateral and 37 had bilateral atrophy. Sperm count and motility decreased in accordance with testicular volume. The lowest mean sperm counts and lowest mean motility percentages were found in patients with bilateral testicular atrophy. An increase in mean FSH and LH was also significantly linked to decreasing testicular volume; the increase in the former was not related to a drop in sperm count. A significant correlation was found between testicular volume and spermatogenesis and testicular volume is considered to be a reliable indicator of testicular function.