We assessed whether the smoking habits of parents around the time of conception affects the likelihood of the offspring being male or female. We found that the offspring sex ratio (male to female) was lower when either one or both of the parents smoked more than 20 cigarettes per day compared with couples in which neither of the parents smoked. We found the lowest sex ratio among children whose mothers and fathers both smoked more than 20 cigarettes per day (p<0·0001). Parental periconceptional smoking might be a contributing factor to a lower male to female sex ratio of offspring.