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Background: The increasing prevalence of overweight and obesity is a major public health concern in many developed countries. Objective: We aimed to describe socioeconomic differences in change in body mass index (BMI; in kg/[m.sup.2]) from age 25 y, assess possible factors behind these differences, and study whether socioeconomic differences in a variety of coronary risk factors can be accounted for by change in BMI. Design: The data come from a cohort study of London-based civil servants (Whitehall II), who participated in the first (1985-1988) and third (1991-1993) phases of the study and were 35-55-y old at phase 1; altogether there were 5507 men and 2466 women. Both study phases included a questionnaire and a screening examination. Results: In men and women, employment grade--the measure of socioeconomic status used in this cohort--was strongly related to BMI gain from age 25 y to phase 3 (25 y apart on average). The lower the grade the larger the gain in BMI. Adjustment for health behaviors reduced the grade differences in BMI gain by [approximately equals] 20%. A substantial part of the grade differences in diastolic and systolic blood pressure and plasma triacylglycerol concentrations could be accounted for by BMI change from age 25 y. Conclusions: Grade differences in BMI change are evident, but many of the determinants of these differences remain unknown. If lower-status persons continue to gain weight more rapidly than higher-status persons, overweight is likely to be of growing importance as a pathway to social inequalities in ill health. Am J Clin Nutr 1999;69:719-26.