Braback L, et al. - Whether sex of the exposed parent and/or grandchild modifies the link between grandmaternal smoking and grandchild asthma, was determined in this cohort study based on linkage of national registries with prospectively collected data over three generations. Based on the ages when medication was purchased, the cohort was grouped into never, early transient (0-3 years), early persistent (0-3 and 4-6 years), and late-onset (4-6 years) phenotypes of childhood asthma. Maternal but not paternal exposure to nicotine before conception was shown to be associated with an increased risk of early persistent childhood asthma, but not other asthma phenotypes. These data may be consistent with a sex-specific mode of epigenetic transfer.