A summary of the work performed by Stanley Garn relating to muscle mass while he worked at the Fels Research Institute is followed by a critical review of the measurement of sarcopenia, which is a deficiency of muscle mass, and its implications for health. The only satisfactory data for total muscle mass come from one cadaver study of a small selected sample. Analyses in relation to health (mortality rates) are limited to indices of muscle mass of which the body mass index (BMI) and arm muscle area or circumference are the most important because large amounts of serial data are available for analysis. The association between low BMI values and increased mortality rates is well established. Multiple studies, some of large samples, indicate that the increased mortality in those with low BMI values at entry is not fully explained by smoking behavior or disease at entry. There are, however, contrary findings from one major study. Associations between arm muscle circumference or area with mortality rates have also been reported. Independent effects of low BMI and low arm muscle values on mortality rates could be due to effects of low fat-free mass, particularly low muscle mass, on host defense mechanisms and risk factors for disease but few relevant data have been reported. © 1994 Wiley-Liss, Inc.