The impact of using different measures of the effect of obesity on mortality across age groups has not been explored. The authors examined this issue by using mortality data from 62,116 men and 262,965 women in the Cancer Prevention Study-I (1960-1972). Measures of effect were calculated separately, by decade of age, for five groups of participants aged 30-79 years. The rate ratio associated with obesity declined with age, from 2.60 (men) and 1.99 (women) for participants aged 30-39 years to 1.24 (men) and 1.15 (women) for those aged 70-79 years. In contrast, the rate differences between obese and reference-weight participants increased with age, from 201 (men) and 112 (women) deaths per 100,000 person-years for those in the youngest decade to 1,379 (men) and 626 (women) deaths per 100,000 person-years for those in the oldest decade. The years of life lost attributable to obesity tended to increase with age but declined for those in the oldest decade. The rate advancement period declined with decade of age. Standardization of estimates to a population changed some age-associated trends. The direction of trends varied regarding the effect of obesity on mortality across age groups, depending on the measure of effect used.