A multiple regression analysis of several factors influencing basal metabolic rate (BMR) was performed using data for 223 subjects from the classic metabolism studies published by Harris and Benedict in 1919. These data had previously been analyzed by Kleiber using metabolic body size, the three-fourths power of body mass, as a predictor of BMR. His prediction equations were separated by sex and each contained components for age and height. Factors in the present analysis included sex, age, height, body mass, and estimated lean body mass (LBM). Lean body mass was found to be the single predictor of BMR. A best estimate prediction equation: BMR(cal/day) = 500 + 22 (LBM) is proposed. The previously presumed influences of sex and age are shown to add little to this estimation.