Longitudinal studies observing working conditions and weight gain are rare. We aimed to study whether weight gain can be predicted by working conditions or changes in them in a 28-year follow-up. The study population consisted of working-aged industrial employees (n = 449). Data on height and weight were collected in clinical examinations in 1973, 1983 and 2001, and information on working conditions by questionnaires. We analysed the impact of changes in physical strain, temporal requirements and indicators of mental strain at work in 1973 and 1983 on weight changes using analysis of variance, logistic regression analysis and linear mixed longitudinal growth model. The latter was done to account for individual variation in temporal weight change over the study period. Weight gain was prominent in the data and about a third of the participants gained at least 15 kg by the final survey. Changes in physical strain and temporal requirements including working overtime hours among men, and experiencing increased working pace among women, were associated with greater mean weight gain and major weight gain. Job efforts and increasing mental strain showed weak associations with weight gain in men. Changes in the physical strain and temporal requirements seem to predict weight gain but working conditions were in general mostly weakly associated with weight gain in this cohort of initially young adults. The results highlight the importance of stable working hours and reasonable workload on healthy weight.