A population study with 6-year follow-up of 6 167 persons aged over 30 was carried out in nine population groups in Southern Finland. Estimation of spondylosis and DISH (Diffuse Skeletal Hyperostosis) was made from lateral chest X-rays. Reliability coefficients (kappa) in the repeat reading of 1 025 films ranged between 0.60 and 0.76. 214 cases of newly developed DISH and 1 080 of spondylosis were observed. With the exception of 4 new cases, all cases of DISH had developed in persons who had had spondylosis at baseline or developed it during the follow-up. The sexual incidence of spondylosis was fairly similar, i.e. 4 cases per 100 person years in both. Prevalence and incidence of spondylosis were highest in rural areas, in persons with strenuous occupations and in the obese. Incidence of DISH was 0.7 cases per 100 person years in men and 0.4 in women. DISH was equally common in all types of population. It was not associated with arduousness of occupation. Obesity and-to a lesser degree-diabetes mellitus and glucose intolerance were associated with DISH. Neither condition was associated with elevated serum calcium, serum cholesterol or bacteriuria. The study supports the concept that DISH is epidemiologically and pathogenetically different from spondylosis deformans.