We performed quantitative analysis of synovial fibrosis in the infrapatellar fat pad in 26 patients who underwent arthroscopically assisted anterior cruciate ligament reconstructions. Twelve patients underwent reconstruction with patellar tendon autografts, and 14 had reconstructions with semitendinosus and gracilis tendon autografts. Synovial samples were obtained at the time of reconstruction from 10 patients and at second-look arthroscopy from all 26 patients. Sections from quick-frozen samples were stained with either hematoxylin and eosin or Fast green and Sirius red. We used sodium hydroxide in absolute methanol to elute the Fast green and Sirius red stains, and the total collagen content of each section was estimated by measuring the optical density of the eluted solution. The volume of each section was determined on a computer using an imaging program, and collagen content per unit of tissue was calculated. Median collagen content was 15.3 micrograms/mm3 for the preoperative samples, 25.1 micrograms/mm3 for the group with patellar tendon autografts, and 27.1 micrograms/mm3 for the group with hamstring tendons autografts. Analysis of preoperative and postoperative paired samples revealed a significant increase in synovial collagen after anterior cruciate ligament reconstruction. We observed increased fibrosis in patients who had pain on exertion or stiffness in squatting after the reconstructive surgery.