About 10,000 tons of chrysotile per year are used in the Federal Republic of Germany for the production of friction materials. During brake repair an unknown number of approximately 300,000 mechanics in automobile service stations are exposed to asbestos dust. In a field study, asbestos fiber concentrations during brake repair were measured. Occupational histories and chest X-rays of brake service mechanics are being examined. Ninety dust measurements in 76 service stations were made by phase contrast microscopy and by scanning transmission electron microscopy. By electron microscopy, extremely fine chrysotile fibers with lengths less than 5 microns were identified in brake drum dust. Fibers with lengths greater than or equal to 5 microns constituted less than 1% of all chrysotile fibers counted in brake drum dust. Short-term asbestos dust exposures were measured by light microscopy in 101 personal samples during blowing out of brakes, and grinding and turning of brake linings. During blowing out of car brakes, as well as during grinding of brake linings, the product of fiber concentration with length greater than 5 microns and sampling time amounted to about 4-5 fibers/ml X min corresponding to a concentration of 10(6) fibers/m3 over 4-5 min. For trucks and buses higher amounts of 5-10 X 10(6) fibers/m3 X min were observed during these operations. From occupational histories of 210 vehicle mechanics, an average duration of employment of mean +/- s = 21 +/- 10 years and a mean cumulative fiber dose of mean +/- s = (0.54 +/- 1.1) X 10(6) fibers/m3 X years were calculated.