BACKGROUND AND AIMS: To describe the clinical findings and surgical treatment of peroneus brevis split. MATERIAL AND METHODS: Two cases of longitudinal split of the peroneus brevis tendon are reported. One of the patients was a healthy middle-aged woman, who had fallen out of a car in a traffic accident and sprained her right ankle. Lateral ankle sprain was diagnosed and treated with a compression bandage. Lateral ankle pain persisted, however, with some swelling in the peroneal tendon region. MRI revealed a longitudinal partial rupture of the peroneus brevis tendon, which was treated surgically 12 months after the trauma. The second case was a 53-year-old woman, who had been suffering from rheumatoid arthritis for 2 years. Chronic pain and swelling in the peroneal tendon region were treated with 6 local corticosteroid injections without significant relief. Preoperative ultrasonography showed effusion of the peroneal tenosynovium, but the operation revealed a longitudinal split in the peroneus brevis tendon. RESULTS: In the first case, a single central peroneus brevis split was repaired with side-to-side suturation. After four weeks with a below-knee cast the patient was allowed to walk freely. At follow-up 12 months postoperatively, she was satisfied, although she still had some exertion pain in her ankle. In the second case, the torn fragment of the peroneus brevis tendon was excised and the ankle was mobilized early. Healing was complicated by a wound fistula, which was treated with antibiotics. Subluxation of the peroneus longus tendon necessitated a reoperation, which revealed a rerupture and a defect of the peroneus brevis tendon. The subluxation was repaired and the ruptured tendon ends were revised, followed by four weeks of below-knee cast immobilization, after which the patient was allowed to walk freely. The outcome was good. CONCLUSION: Peroneus brevis split easily goes unrecognised or misdiagnosed. It must be considered in patients with a history of single or recurrent ankle sprain or a chronic inflammatory disease. Lateral ankle pain, diffuse or local swelling in the peroneal tendon region, and a stable or instable ankle with no peroneal weakness are the main symptoms and findings. MRI is the most exact method for diagnosing tendon split. Surgical treatment usually gives good results.