OBJECTIVE To estimate the extent to which increasing BMI may explain the rise in type 2 diabetes incidence in British men from 1984 to 2007. RESEARCH DESIGN AND METHODS A representative cohort ratio of 6,460 British men was followed-up for type 2 diabetes incidence between 1984 (aged 45–65 years) and 2007 (aged 67–89 years). BMI was ascertained at regular intervals before and during the follow-up. RESULTS Between 1984–1992 and 1999–2007, the age-adjusted hazard of type 2 diabetes more than doubled (hazard ratio 2.33 [95% CI 1.75–3.10]). Mean BMI rose by 1.42 kg/m2 (95% CI 1.10–1.74) between 1984 and 1999; this could explain 26% (95% CI 17–38) of the type 2 diabetes increase. CONCLUSIONS An appreciable portion of the rise in type 2 diabetes can be attributed to BMI changes. A substantial portion remains unexplained, possibly associated with other determinants such as physical activity. This merits further research.