The strength and endurance of the trunk muscles was studied in relation to the extent of earlier low-back trouble (LBT) in a homogeneous, and occupationally active group. Twenty-four female and 53 male postmen with an occupational seniority of more than 2 years took part in the investigation. The cumulative lifetime prevalence, the one-year and the point prevalence of LBT were 67%, 62%, and 4% in females and 55%, 52%, and 0% in males. The rates are higher than in a representative Danish population 40 years old. Anthropometrical measurements and isometric strength (MVC) in trunk flexors and extensors were recorded. The flexibility of the spine, hip and knee joints, the fingertip-floor distance, and the restricted extension of the knee were evaluated. The isometric endurance in the trunk extensors was measured by two methods: 1) prone with the unsupported trunk in a horizontal position and the legs and hips fixated to a couch; and 2) standing, at 60% MVC. The participants were divided into three groups according to the extent of previous LBT, Group I: LBT to a degree that made work impossible, Group II: LBT experienced but not to such a degree that work was hindered, and Group III: LBT never experienced. The main findings were that the isometric endurance time of the trunk extensors was shorter in group I than in II and III, while the trunk muscle strength, anthropometrical measures and joint flexibility were independent of the persons' earlier low-back episodes. Differences in the distribution of ST and FT muscle fibres are suggested as an explanation of the endurance difference.(ABSTRACT TRUNCATED AT 250 WORDS)