Background: Maternal smoking negatively affects bir th weight, breast milk volume and growth at one month of age. Aim: To study the effect of maternal smoking on the growth of three month old infants. Subjects and methods: Ten smoking and 10 non smoking mothers with one month old children, under exclusive breast feeding, were selected for the study. Urinary cotinine levels in the mothers and children were measured to assess smoking. Two months later; children were reassessed. Results: Smoking mothers consumed a mean of 6.5 cigarettes/day. Compared to non smokers, urinary continine levels were higher in these mothers (60 +/- 22 and 1428 +/- 716 ng/ml respectively, P < 0.002) and their offspring (21 +/- 20 and 156 +/- 101 ng/ml respectively p < 0.001). Compared with smokers, at three months of age, children of non smoking mothers had a higher average weight (5829 +/- 498 and 6325 +/- 427 g, P < 0.02). Conclusions: Measurement of urinary continue levels is a reliable and objective measure of maternal smoking and nicotine transfer to the offspring. It also allows the assessment of passive smoking. Maternal smoking adversely affects child growth at three months of age.