Abstract Modic changes are vertebral bone marrow signal intensity changes adjacent to the endplates of the degenerated intervertebral discs in magnetic resonance imaging (MRI). This study evaluated the prevalence and the determinants of Modic changes and their association with low back pain symptoms in an occupational cohort of middle-aged Finnish men. The prevalence and the natural course of Modic changes were assessed over a 3-year follow-up period among sciatica patients. Finally, in a patient population, the characteristics of bone marrow changes in MRI were compared to the imaging findings in CT. The prevalence of Modic changes was 56% in an occupational cohort of middle-aged males. Besides age, the determinants of Modic changes and disc degeneration were different. Weight-related factors, which add to the load of the lumbar spine, were associated with Modic changes, whereas whole-body vibration was associated with severe disc degeneration. The prevalence of Modic changes among sciatica patients was 65%, type II change being the most frequent. During the 3-year follow-up, 14% of changes converted to another type, while the incidence of new Modic changes was 6%. Among middle-aged working males, Modic changes located at L5–S1 and type I Modic changes were more likely to be associated with pain symptoms than other types of Modic changes or changes located at other lumbar levels. Thirty-eight percent of the endplates with Modic changes had sclerosis in CT. Of specific Modic types, mixed I/II and II/III associated significantly with endplate sclerosis. Endplate sclerosis was not detected in MRI. In conclusion, Modic changes are a common MRI finding both among patients and middle-aged working males. In addition to age, weight-related factors seem to be important in the pathogenesis of Modic changes. Modic changes can convert from one type to another and type II changes may be less stable than previously assumed. A considerable proportion of Modic changes are sclerotic as observed in CT. Modic changes were always found in combination with a degenerative intervertebral disc and thus they are assumed to be a specific phenotype of degenerative disc disease. Finally, Modic changes may be painful – especially when located at L5–S1 and type I changes.