There is no clear link between obesity and prostate cancer incidence but an association has been reported between obesity and fatal prostate cancer. We report on two prospective cohort analyses on (i) the incidence of prostate cancer in relation to obesity in a cohort of men with no previous cancer, and on (ii) the stage distribution and prostate cancer specific mortality in relation to obesity among men with prostate cancer. The "Diet, Cancer and Health" prospective cohort study was established in Denmark in 1993-1997 and accrued 26,944 men aged 50-64 years. Data were extracted on height, weight, body mass index (BMI), waist circumference and body fat percentage. Information on cancer incidence and deaths were obtained by record linkage with the Danish Cancer Register and the Danish Death Register. The incidence rate of prostate cancer was similar or slightly lower in obese men compared with nonobese men, but obese men tended to be diagnosed with more advanced prostate cancer. The proportion of Stage 3-4 cancers was 37% in the lowest BMI quartile and 48% in the highest (p = 0.006). Obese men with prostate cancer had higher prostate cancer specific mortality. The hazard ratio comparing the highest and the lowest quartiles of BMI was 1.48 (95% confidence interval: 1.06-2.05; p-value for trend: 0.002). The association was attenuated but not eliminated by statistical adjustment for stage, and the data are suggestive of a stage-independent causal pathway where prostate cancer in obese men has higher fatality, even in early-stage disease.