Conservative treatment is currently recommended for most isolated posterior cruciate ligament (PCL) injuries in athletes. However, it is not known whether conservative treatment is applicable even in high performance athletes with isolated PCL injury. The results in two extremely high performance athletes, professional baseball players with isolated acute PCL injury treated conservatively are reported. A catcher and an out fielder, who were regular players, hurt their knees in baseball games. Magnetic resonance images of the knee detected complete PCL rupture. Following a carefully guided physical therapy program, a 3-week period of immobilization of the knee in full extension was achieved with a knee brace, while performing hard quadriceps muscle strengthening exercise, and then running exercise was started. Six to eight weeks after injury, they were able to return fully to their original sporting activity despite tibial posterior translation on posterior drawer test, and to sustain this activity over 2 years. Switching of weight-bearing to non-weight-bearing in a deep knee flexion is considered to contribute to subjective instability in athletes with PCL-deficiency. Probably because our cases, even though extremely high performance athletes were infrequently subjected to such a situation while playing baseball, they were able to return to their pre-injury level of athletic performance without severe subjective instability through conservative treatment.