The aim of this study was to analyse differences in smoking between men and women, and between different age groups of Finnish adults from 1978 to 1990. The data were collected by questionnaires from mail surveys of representative nationwide samples of 15-64-year-olds from 1978, annually. Response rates varied between 68% (1985) and 86% (1978), averaging 79%. Those who were less than 20 years old were excluded from this study because smoking as a habit is well-established in those aged 20 years and over. Male smoking decreased (from 37 to 33%) and female smoking increased (from 17 to 20%) during the study period. The proportion of smokers among the youngest men (20-29-year-olds) had decreased clearly, and after the mid-1980s the proportion of male smokers was highest in the middle-age group (30-49-year-olds). The proportion of smokers among the middle-aged women increased, but the proportion of smokers was highest in the youngest age group. Gender difference in smoking was largest in the oldest age group.