AIMS: To examine the association between self reported physical workload and low back pain (LBP) in younger twins. To investigate whether genetic factors interact with physical workload in relation to LBP. METHODS: A twin control study was performed within a population based twin register using 1910 complete monozygotic (MZ) and same sexed dizygotic (DZ) twin pairs aged 25-42 and discordant for LBP. LBP in the affected twins was divided into two groups: "LBP for 30 days during the past year". Physical workload was divided into four categories: "sitting", "sitting/walking", "light physical", and "heavy physical". Data were analysed in a matched design using conditional logistic regression. MZ and DZ twins were analysed separately and together in order to determine possible genetic influences in relation to physical workload and LBP. RESULTS: Statistically significant graded relations were found for increasing workload and LBP of longer duration but not for LBP of shorter duration (