Coca-leaf chewing results in absorption of part of its cocaine content. Following absorption, cocaine and/or its stable metabolic product benzoylecgonine (BZE) may enter the chewer's hair follicles and subsequently be incorporated into the hair shafts. This article reports that a recently developed radioimmunoassay is capable of detecting BZE in hair samples from ancient, spontaneously mummified human remains. Results are provided from tests on hair samples of 163 individuals, representatives of populations from seven different cultures living at coastal and low valley sites in northern Chile during the past 4,000 years. These indicate that coca-leaf-chewing practices began in this area about 2,000 years ago. The practice seems to have been common in several subsequent cultural groups. In one of these—Maitas Chiribaya—the majority of both adult men and women indulged in this practice. Coca-leaf-chewing women probably transferred BZE to their fetuses and nursing infants.