The effect of postoperative immobilization on the length, stiffness, and structure of connective tissue after radiofrequency probe shrinkage was examined in a rabbit patellar tendon model. Tendon lengths were measured before, immediately following, and 2, 4, and 8 weeks following heat application, with the contralateral tendon as a control. The animals were randomly assigned to three groups. In Group I, the controls, the animals were allowed free caged activity for 8 weeks. In Groups II and III, the experimental limb was immobilized at 15 degrees of knee flexion for 2 and 4 weeks, respectively, after which the animals were allowed 6 and 4 weeks of free caged activity. Changes in tendon length, stiffness, and cross-sectional area were compared using repeated measures ANOVA and differences between groups examined using Tukey's post-hoc analysis. Patellar tendon lengths were increased in all 3 groups at 8 weeks (P< .001). Tissue elongation was less extensive in group III compared to group I at 8 weeks (P< .001), while tissue cross-sectional area was decreased in both groups II and III compared to group I at 8 weeks (P< .001). All groups demonstrated decreases in tensile strength when compared to controls (P< .001). These findings suggest that postoperative immobilization is important following thermal shrinkage of connective tissue, as early activity can lead to stretching of the heat-modified tissue.