The macroanatomy of the clinically important lumbar muscles has recently been investigated in detail, whereas the information about their microscopic structure in healthy persons is still scanty. In this study we have analysed lumbar multifidus and erector spinae muscles from 21 previously healthy persons who died suddenly and were of working age (range 23-65 years, mean 44.7 years). The microscopic structure of myofibers within the lumbar muscles was found to be regionally uniform, which renders in vivo biopsies representative of the whole muscle bulk. Somewhat surprisingly, age did not significantly influence fiber type composition, fiber size, or proportion of nonmuscular tissue. There was a slight predominance of type 1 fibers. The size of type 1 fibers (mean lesser diameter 54.0 microns) corresponded to that in other skeletal muscles, with no significant sex difference (55.1/51.6 microns male/female). Selective type 2 fiber atrophy was a common finding in both sexes, but significantly less so in men (mean diameter 38.8/28.4 microns male/female). We suggest that the values of fiber type proportions and fiber size in the deep multifidus presented in this study can be used as reference values for healthy adults in the present society with limited physical activity.