105 factory workers (38 females and 67 males) have been questioned about their frequency of back pain. 60% of the females and 61% of the males have previously experienced episodes of back pain. 21% of the females and 37% of the males have been absent from work due to back pain. The incidence of back pain is not related to age, height, sort of work, or isometric muscle strength of the back (IS). For the males the incidence rises with increasing weight, i.e. combination of height and obesity, but is not related to any two single factors. For the females there is no correlation between the incidence of pain and weight. IS is correlated to height and age in the males but not in the females. Standards for IS are presented and suggested as a guide to evaluation of the working capabilities of individual subjects with back pain.