Body mass index has important predictive value for mortality and morbidity both in normal subjects and in those suffering from particular pathologies. However, body mass index was introduced as a measure of body fat, which might not be expected to have such wide implications for various pathological conditions. We argue here that body mass index may actually be a measure for longevity. Our arguments are based on a well-established allometric scaling law for physiological time. The time between heart beats, the time between respirations, and longevity all scale as body weight to the 1/4 power in mammalian species ranging from shrews to blue whales. We find that body mass index also scales with body weight to the 1/4 power in humans from birth to one year of age, and again from approximately 5 to 17 years of age. On the assumption that in these two growth phases humans scale as do species, we postulate that body mass index is a measure of longevity.