From January 1999 to May 2005, 25 patients (15 males and 10 females; age range, 18-70 years; mean, 42 years) who demonstrated clinical and radiographic evidence of atlantoaxial instability underwent C1 lateral mass and C2 pedicle screw internal fixation with or without fusion at our Orthopedic Unit. The cause of instability was: 13 patients, traumatic fracture; three patients, rheumatoid arthritis; two patients, rotatory subluxation; two patients, congenital malformation; five patients, failed previous surgery. A mean follow-up of 16 months was obtained (range, 4-48 months). Mean operative time was 107 minutes (range, 80-141 minutes). No patient received a blood transfusion. No patient experienced worsening neurological function related to the procedure postoperatively or at follow-up. No other postoperative complication was observed. All patients were relieved from axial pain. Screw placement and reduction were achieved satisfactorily in all patients. Each patient showed evidence of solid fusion after 12 months by plain radiography and dynamic films. During follow-up, no complications were observed related to the bone graft or the screw rod. We suggest that C1 lateral mass and C2 pedicle screw internal fixation is a reliable method to repair atlantoaxial instability.