Chronic pain in the region of the Achilles tendon is a common problem and often a sign of progressive degeneration of the tendon which may lead to its rupture. We studied the clinical course and sonograms in 36 patients with achillodynia to find a prognostic parameter enabling us to estimate the risk of rupture. The patients were evaluated clinically for swelling and tenderness and by high-resolution real-time sonography. The sonograms were graded according to the tendon thickness as normal (< 6 mm), minimal (6-8 mm), moderate (8-10 mm) to high-grade (> 10 mm) in the sagittal diameter of the transverse section, and alterations of echotexture were described as diffuse, circumscribed, or inhomogenous. At the time of the primary investigation, we found thickening and alterations of the echotexture in 33 of 72 tendons. In 48 tendons we found pain and local or diffuse swelling in the Achilles tendon region (sensitivity 0.58, specificity 0.79). After a follow-up of 48 +/- 8 months, 7 tendons had ruptured spontaneously. Analysis of the sonograms of the patients taken prior to the rupture showed a high-grade thickening in 4 cases, moderate thickening in 2 cases, and a diameter between 6 and 8 nm in one patient. In no case did we find a rupture of a tendon primarily classified as normal. Patients without sonographic changes exhibited a significantly better clinical outcome following conservative treatment. Sonography was found to be a valuable tool for determination of the tendon's thickness and echotexture. In 28% of our patients with thickening, circumscribed lesions of the echotexture, and chronic pain, a spontaneous rupture occurred.