Objective: To evaluate any association between obesity in middle age, measured by body mass index and skinfold thickness, and risk of dementia later in life.

Design: Analysis of prospective data from a multiethnic population based cohort.

Setting: Kaiser Permanente Northern California Medical Group, a healthcare delivery organisation.

Participants: 10,276 men and women who underwent detailed health evaluations from 1964 to 1973 when they were aged 40-45 and who were still members of the health plan in 1994.

Main outcome measures: Diagnosis of dementia from January 1994 to April 2003. Time to diagnosis was analysed with Cox proportional hazard models adjusted for age, sex, race, education, smoking, alcohol use, marital status, diabetes, hypertension, hyperlipidaemia, stroke, and ischaemic heart disease.

Results: Dementia was diagnosed in 713 (6.9%) participants. Obese people (body mass index > or = 30) had a 74% increased risk of dementia (hazard ratio 1.74, 95% confidence interval 1.34 to 2.26), while overweight people (body mass index 25.0-29.9) had a 35% greater risk of dementia (1.35, 1.14 to 1.60) compared with those of normal weight (body mass index 18.6-24.9). Compared with those in the lowest fifth, men and women in the highest fifth of the distribution of subscapular or tricep skinfold thickness had a 72% and 59% greater risk of dementia, respectively (1.72, 1.36 to 2.18, and 1.59, 1.24 to 2.04).

Conclusions: Obesity in middle age increases the risk of future dementia independently of comorbid conditions.