The objective of this retrospective study was to correlate the Bado and Jupiter classifications with long-term results after operative treatment of Monteggia fractures in adults and to determine prognostic factors for functional outcome. Of 63 adult patients who sustained a Monteggia fracture in a ten-year period, 47 were available for follow-up after a mean time of 8.4 years (5 to 14). According to the Broberg and Morrey elbow scale, 22 patients (47%) had excellent, 12 (26%) good, nine (19%) fair and four (8%) poor results at the last follow-up. A total of 12 patients (26%) needed a second operation within 12 months of the initial operation. The mean Broberg and Morrey score was 87.2 (45 to 100) and the mean DASH score was 17.4 (0 to 70). There was a significant correlation between the two scores (p = 0.01). The following factors were found to be correlated with a poor clinical outcome: Bado type II fracture, Jupiter type IIa fracture, fracture of the radial head, coronoid fracture, and complications requiring further surgery. Bado type II Monteggia fractures, and within this group, Jupiter type IIa fractures, are frequently associated with fractures of the radial head and the coronoid process, and should be considered as negative prognostic factors for functional long-term outcome. Patients with these types of fracture should be informed about the potential risk of functional deficits and the possible need for further surgery.