The fatty acid composition of fat tissue is a valid indicator of the fatty acid composition of the diet over the preceding 1 to 3 yr. Here we describe a rapid method for sampling of buttock fat without anesthesia. On average, 35 mg can be obtained using a common blood- sampling system. Sampling of 40 subjects takes about 3 h. The procedure caused no more anxiety or discomfort than a routine venipuncture. Application in about 500 subjects has as yet yielded no infectious or other complications. With regard to the fatty acid composition of the biopsy, sampling site (left versus right buttock) was not found to be a source of error. The biopsies could be stored for periods up to 1.5 yr without changes in the fatty acid profile. With this method one can obtain biopsies from a large number of subjects and determine objectively the long-term fatty acid composition of their diet.