It has been suggested that certain health behaviours, such as smoking, may operate as mediators of the well-established inverse association between IQ and mortality risk. Previous research may be afflicted by unadjusted confounding by socioeconomic or psychosocial factors. Twin designs offer a unique possibility to take genetic and shared environmental factors into account. The aim of the present national twin Study was to determine the interrelations between IQ at age 18, childhood and attained social factors and smoking status in young adulthood and mid-life. We studied the association between IQ at age 18 and smoking in later life in a population of 11 589 male Swedish twins. IQ was measured at military conscription, and data on smoking and zygosity was obtained from the Swedish Twin Register. Information on social factors was extracted from censuses. Data on smoking was self-reported by the twins at the age of 22-47 years. Logistic regression models estimated with generalised estimating equations were used to explore possible associations between IQ and smoking among the twins as individuals as well as between-and within twin-pairs. A strong inverse association between IQ and smoking status emerged in unmatched analyses over the entire range of IQ distribution. In within-pair and between-pair analyses it transpired that shared environmental factors explained most of the inverse IQ-smoking relationship. In addition, these analyses indicated that non-shared and genetic factors contributed only slightly (and non-significantly) to the IQ-smoking association. Analysis of twin pairs discordant for IQ and smoking status displayed no evidence that non-shared factors contribute substantially to the association. The question of which shared environmental factors might explain the IQ-smoking association is an intriguing one for future research.