Blue-collar and white-collar employees in the metal industry were studied for leisure time physical activity, smoking, the body mass index, stress symptoms, and low back disorders by questionnaire, interview, and clinical examination. Measurements were made three times at 5-year intervals. The initial sample of 902 was stratified for age group, sex, and occupational class. Six hundred seven subjects took part in both re-examinations. Based on the quality, strenuousness, duration, and frequency of leisure time physical activity three scores were constructed: total activity outside work, exercise, and strenuous (500 kcal/h) activity. Low back morbidity was measured as the abundance of symptoms during the past year and clinical findings as assessed by a physiotherapist. No associations between physical activity and back disorders were observed at baseline. In men, the mean exercise activity during the first 5-year follow-up was moderately inversely associated with the back symptoms and findings at the end of the follow-up, when the relevant morbidity score at the second examination, age, and occupational class were allowed for in multiple regression analysis. The effects persisted when data on smoking, the body mass index, and stress symptoms were added into the models. Strenuous activity predicted the change only in the clinical findings, and the association was reduced when the other lifestyle factors were accounted for.