Age- and sex-specific percentile distributions for mid-upper arm muscle area (AMA), mid-upper arm fat area (AFA), subscapular skinfold thickness, and sum of triceps and subscapular skinfold thickness for United States adults (18-74 yr) were developed from cross-sectional data collected by the National Center for Health Statistics (NCHS) during the Health and Nutrition Examination Survey, 1971-1974 (NHANES I). Sample weighting factors were applied to the data to assure the production of the identical data set used to calculate weight-for-height percentiles by NCHS. Smoothed curves developed from the percentile distributions for AMA and AFA are also presented. Arm muscle area and AFA were calculated from the Health Examination Survey of 1960-1962 (HES) which made possible an age-cohort analysis for age-related changes and secular trends. Secular differences were found for arm muscle area for both men and women, while men differed little in fat-related anthropometric parameters. The secular change in arm fat area for women could be explained by the greater amount of fat that surrounded a larger arm, since the ratio of AFA to AMA did not change between the two surveys. Age-cohort analysis revealed increases in AMA with age rather than the apparent decrease observed for the older age groups in the cross-sectional data. Reference values developed from the most recent cross-sectional data are the most appropriate for the nutritional assessment of United States adults, because each age cohort can be seen to display different anthropometric characteristics as well as age-related changes in these parameters.