The possible effects of cigarette smoking on sperm quality were investigated in a blind study of 333 healthy male volunteers 19 to 40 years of age. All subjects with clinically proven abnormalities, operations, or inflammatory diseases of the genital organs, exposure to radiation exceeding routine x-ray examination, or medication were excluded, and the data from 150 smokers, 37 ex-smokers, and 52 never-smokers were evaluated. There was a small but insignificant decrease in sperm density from never-smokers to ex-smokers and to smokers. Eight smokers (6%) had sperm counts less than 1 X 10(6)/ml, whereas none of the ex-smokers or never-smokers fell into this category. This difference, however, is not statistically significant (P = 0.084). No statistically significant effect of smoking habits on sperm density, motility, and morphologic features was detected. It is concluded that smoking does not affect the motility and morphologic features of sperm in healthy adult men. The diminished sperm density in heavy smokers may be related to factors associated with smoking.