The prevalence rate of musculoskeletal problems, especially low back pain and severe low back pain in a randomly selected sample of 1,773 construction workers was studied. Its relationship to physical and psychosocial factors was analyzed. The workers answered a postal questionnaire. Workload was measured by means of eight manual materials handling indices and ten psychosocial indices, based on results from factor analyses. The 1-year prevalence rate of low back pain was 54% and of severe low back pain 7%. The relationship to heavy manual materials handling differed with age in such a manner that it could be interpreted as a healthy worker effect. Between severe low back pain and both stooping or kneeling a dose-response relationship was found. The most prominent of the psychosocial factors associated with low back pain and severe low back pain were the stress index and the psychosomatic and psychic indices. The age-standardized prevalence rate ratio of low back pain was 1.6 (95% confidence interval 1.4-1.8) and for severe low back pain 3.1 (95% confidence interval 2.3-4), when workers reporting "high" stress were compared to workers reporting "low" stress.