Comparison of semen quality was undertaken between 60 smoking and 50 non-smoking men attending infertility investigation. As far as possible, other factors which might influence semen quality were eliminated. The men were allocated into groups according to sperm density and number of cigarettes smoked per day. The percentage of motile sperm in heavy smokers with oligozoospermia and the total sperm count were lower, the difference being significant at the 1% and 5% levels, respectively. No correlation was found in percentage of sperm with morphologically abnormal heads, midpieces or tails. Heavy smoking may have a detrimental effect on the motility of sperm. Further studies are needed to explain the mechanisms by which smoking affects spermatogenesis.