OBJECTIVE: To assess seminal plasma insulin-like growth factor-I (IGF-I) levels in cigarette smokers with idiopathic oligoasthenoteratozoospermia (iOAT). PATIENTS AND METHODS: In all, 110 men were divided into fertile healthy non-smokers, fertile smokers, infertile non-smokers with iOAT and infertile smokers with iOAT. Semen was analysed, and seminal cotinine and seminal IGF-I levels estimated. RESULTS: There were significantly lower seminal IGF-I levels in the smokers and in men with iOAT than in controls, and in both iOAT groups. Smokers, either fertile or with iOAT had significantly lower levels than in controls in mean semen volume, sperm production index, percentage of motile sperms, rapid linear forward progressive motility, linear velocity and sperm normal forms. Smokers with iOAT had significantly lower levels than non-smokers with iOAT in mean sperm production index, rapid linear forward progressive motility and linear velocity. In smokers, seminal cotinine was significantly and negatively correlated with both seminal IGF-I and sperm motility, while seminal IGF-I was positively correlated with the percentage of motile spermatozoa. CONCLUSION: Smoking effects on sperm variables could be mediated by decreased seminal IGF-I.