Anterior cruciate ligament (ACL) fibroblasts obtained from beagle dogs were cultured in basal medium containing different concentrations of 1 to 10-3 µM 5α-dihydrotestosterone (DHT) and in basal medium itself as a control. It was demonstrated that DHT promoted cell proliferation activity, expression of androgen receptor, and collagen synthesis in ACL fibroblasts as compared with control. These results suggest that sex hormones are involved in the sex difference seen in ACL rupture of dogs.