We report the results of a prospective study of the incidence of algodystrophy following Colles' fracture in 60 patients, using sensitive or quantitative techniques for the assessment of each feature of the syndrome. Nine weeks after fracture, 24 patients had evidence of vasomotor instability, 23 had significant tenderness of the fingers and 23 had lost finger movement. These three abnormalities were significantly associated (p less than 0.001). Swelling was also significantly associated with these three variables. There was no statistical association between the occurrence of algodystrophy and the patient's age or sex, the severity of the fracture, the number of reductions performed or the adequacy of the reduction. We conclude that algodystrophy constitutes a precisely definable and quantifiable syndrome which is more common than has been suspected.