We prospectively determined the frequency of atlantoaxial subluxation in a group of patients with systemic lupus erythematosus (SLE) and analyzed its relationship with tendinous laxity, Jaccoud's syndrome and other features of the disease. Five of 59 patients (8.5%) had atlantoaxial subluxation. No patient presented atlantoaxial subluxation in neutral lateral cervical radiographs but all 5 had anterior atlantoaxial subluxation in full flexion films; one patient also had lateral subluxation. The 5 patients with atlantoaxial subluxation were compared with the remaining 54. Mean SLE disease duration was longer in patients with atlantoaxial subluxation (12 years) than in those without (6.6 years) (p less than 0.01). Jaccoud's syndrome, patellar tendon elongation and articular hypermobility were significantly more frequent in patients with atlantoaxial subluxation. The presence or history of arthritis failed to distinguish patients with and without atlantoaxial subluxation, while chronic renal failure and increased serum parathyroid hormone levels were significantly associated to the presence of atlantoaxial subluxation. We suggest that atlantoaxial subluxation is further evidence of tendinous alterations seen in patients with SLE.