To evaluate the relationship between genetic factors and back pain, 5029 MZ and 7876 DZ twin pairs were investigated by a questionnaire. The data were analyzed with regard to physical symptoms, eg, back pain, and certain work conditions. Pain was reported by about 17% of the male and 13% of the female twins in the material. The occurrence of back pain was strongly associated with work load, especially among the male twins. Twin concordance regarding back pain was considerably higher in MZ than in DZ twins. Of four groups who were studied, statistically significant differences between the two twin categories were observed in three cases. This result supports the assumption that a relationship exists between genetic factors and the occurrence of back pain. Official registrations for sick-leave pay in a sample of 247 twins confirmed the interview information on back pain. Just a few cases were wrongly classified, ie, they were sick-listed because of back pain but had stated no back pain in the questionnaire.