Using biceps and triceps skinfolds and upper arm circumference, areas of fat and lean tissue at the arm can be approximately determined based on a cylindrical assumption. Based on the first Zürich growth study a longitudinal analysis of estimated fat and lean areas is undertaken. Area may often be a more meaningful parameter than width in development, due to the increasing width of the limbs stretching the layer of fat. When comparing these quantities with the body mass index, correlations in boys were higher for lean area than for fat area, and about equally high for both measures in girls (around 0.8 beyond age 10). The primary goal of this investigation was to quantify the development of lean and fat arm areas from birth to adulthood, and to assess sex differences in this respect. The comparison with results obtained earlier from radiographic data is of particular interest, since such data can rarely be obtained any more, and overall this comparison can be considered encouraging. Lean area develops slowly until the onset of puberty, girls showing a slightly smaller value than boys. The pubertal spurt is very impressive in boys and moderate in girls, timed to age of peak height velocity in both sexes. Fat area changes only minimally in boys older than 16 months, and increases steadily in girls until age 16. The respective velocity curve shows a systematic up and down until age 5, when a gradual increase to a prepubertal fat spurt starts. It is interrupted by a trough in velocity--much more accentuated in boys--at puberty, and followed by a postpubertal fat spurt. When studying subgroups of subjects with a relatively high or relatively low adult body mass index, the heavy group differed relatively more in terms of fat area than lean area. Girls heavy as adults increase their fat area consistently more from about 5 years onwards. Boys later heavy show a qualitatively different pattern in puberty, with accentuated pre- and postpubertal fat spurts, but also a strong trough to a negative velocity in between.