The archaeological evidence of ancient cranial surgery is limited to cases of trepanation and cauterization. I report here on the only known case of cranial surgery in direct association with the osseous image of a nontrauma-induced soft tissue lesion (sinus pericranii). This case, from Alameda County, California (Late Middle Period, ca. 300–500 AD), is the earliest and only definitive evidence of invasive surgery from prehistoric North America. Because this individual presents the only bony evidence of cranial surgery other than trepanation or cauterization, it contributes substantially to our extremely limited understanding of medical practices in preliterate societies. © 1995 Wiley-Liss, Inc.