The effects of immobilization on healing collogenous tissue treated with thermal shrinkage are investigated in an in vivo rabbit model. Thermal shrinkage was performed on 45 of 50 medial collateral ligaments in 25 mature New Zealand rabbit knees. In half of the knees, the joints were immobilized. Animals were killed at 0, 3, 6, and 9 weeks postoperatively. Failure loads of ligaments were determined, and morphalogic changes were evaluated by electron microscopy. The failure load of shrunken ligaments was significantly lower than that of the intact ligaments on the day of operation (P < .05). Shrunken ligaments reached their highest failure loads in the third week within the first 9 weeks. The immobilized ligaments remained weaker than the mobile group, but this difference was statistically significant only in the ninth week (P < .05). Immobilization seems to hove negative effects on the healing ligament. Ideal timing for remobilization is still controversial, and abandoning immobilization protocols for longer than 3 weeks should be considered.