Tear of the peroneus longus tendon can be difficult to diagnose and treat. Fourteen cases of clinically proven tears are reported. Patients ranged in age from 31 to 63 years. There were 10 men and four women. The onset of symptoms was acute in eight cases. Chronic onset with slowly increasing pain occurred in six cases. Despite acute onset of symptoms, only one patient was diagnosed within 2 weeks of his injury. The others had symptom duration ranging from 7 to 48 months. Twelve tears were located distally as the tendon turned into the cuboid groove. As os peroneum visible on x-ray was present in six cases, absent in seven cases, and cartilaginous in one case. The os peroneum was involved in the tear in three cases. Excision of part or all of the os peroneum was performed in four cases with a bridging tendon graft required in one case. A plantaris tendon graft was required in one case in which an os peroneum was absent. Patients with acute onset of symptoms tended to fare better than those with chronic onset of symptoms, regardless of the length of time from onset to time of surgery. Associated pathology in the peroneus brevis tendon was common, occurring in nine cases. These patients seemed to have a better surgical outcome than those with only a tear of the peroneus longus tendon. Associated findings affected diagnosis by masking symptoms but did not alter the outcome of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)