Eighty patients with grade III lateral ligament ruptures were treated either with total immobilization in a walking plaster cast or early mobilization in a stabilizing orthosis. The criterion for entrance was a talar tilt of more than 9 degrees and an anterior translation of more than 10 mm at stress radiography, a previously stable ankle, and a contralateral ankle showing normal stress radiographic values. Ninety-one percent of the patients were evaluated at 7 weeks, 3 months, and 1 year postinjury. While functionally treated patients reached normal mobility and resumed work and sports earlier than immobilized patients there were no differences between the treatment groups in ankle stability or symptoms during activity after 1 year. Ninety-five percent of the ankles in either group were mechanically stable after treatment. Residual symptoms were present 1 year postinjury in 13% of the functionally treated ankles and in 9% of the cast-mobilized ankles. In lateral ankle ligament ruptures causing gross mechanical instability early mobilization results in a better early functional result; however, at 1 year postinjury there was no statistically significant difference in outcome as compared to cast-immobilized ankles.