To identify the cause of rupture of hand tendons in patients with rheumatoid arthritis, we studied the underlying ultrastructural changes of the collagenous fibril systems. Samples of the flexor digitorum superficialis (n = 12) and the extensor digitorum communis (n = 20) were taken during tenosynovectomy. Tendons dissected at necropsy (n = 30) served as controls. Specimens were analysed by transmission and scanning electron microscopy. Interfibrillar dysplastic fibrils, "Luse bodies", and intracellular collagen were found in rheumatoid tissues. The diameters of collagen fibrils were significantly reduced compared with the control group (p < 0.01). The duration of the disease usually correlated well with the ultrastructural collagenous lesions. To provide optimum conditions for restoration of rheumatoid hand tendons, early synovectomy in rheumatoid patients seems warranted from the ultrastructural point of view. The alterations in collagen may explain the inadequate function of the hand tendons in patients with rheumatoid arthritis.