Sixty patients with acute rupture of the Achilles tendon were included in a prospective study and assigned randomly for operative treatment using a 4-string suture with either a new mobile cast (30) or a rigid below-knee cast (30). All patients were evaluated clinically for 1 year after surgery. During surgery, a radiographic monitor was placed in the Achilles tendon above and beneath the site of rupture. Radiographic evaluation of tendon behavior during healing was also performed on all patients for 1 year. Major complications were 1 rerupture in the mobile cast group and 2 reruptures and 1 infection in the rigid cast group. There were fewer minor complications in the mobile cast group. More patients in the mobile cast group resumed sports activities at the same level as before the rupture than in the rigid cast group. They also had better recovery of normal ankle movement plus faster and better recovery of plantar flexion strength. Fewer patients in the mobile cast group had calf atrophy, and fewer had problems 1 year after the accident. Furthermore, patients treated with the mobile cast had a statistically significant shorter sick leave. Radiographic evaluation of the tendon monitor showed significantly less elongation of the tendon 1 year after rupture for patients in the mobile cast group. Operative treatment with a 4-string suture and use of a postoperative mobile cast proved safe and convenient and preferable to treatment with the traditional rigid below-knee cast.