Treatments for acute distal radioulnar joint (DRUJ) instability with distal radius fracture vary from conservative to operative treatment, although it seems to be no consensus regarding which treatment is optimal. This prospective randomized study was designed to compare the clinical outcomes for operative and conservative treatment of acute DRUJ instability with distal radius fracture, according to the presence or absence and type of ulnar styloid process fracture and the degree of its displacement. Between July 2008 and February 2013, we enrolled 157 patients who exhibited an unstable DRUJ during intraoperative manual stress testing (via the ballottement test) after fixation of the distal radius. Patients were classified according to the type of the ulnar styloid process fracture, using preoperative wrist radiography, and each group was divided into subgroups, according to their treatment method. We then compared the clinical outcomes between the conservative and operative treatments, using their range of motion; Disabilities of the Arm, Shoulder, and Hand score; modified Mayo wrist score; and grip strength. At 3 months after surgery, among patients without ulnar styloid process fracture, the flexion-extension range was 79 ± 15° after supination sugar-tong splinting (group A-1), 91 ± 14° after DRUJ transfixation (group A-2), and 89 ± 10° after arthroscopic triangular fibrocartilage complex repair (group A-3); the operative treatments provided greater joint motion ranges than conservative treatment. The groups with ulnar styloid process fractures at the tip (group B) or base (group C) also exhibited better clinical outcomes after the operative treatments, compared with after the conservative treatment. However, at the final follow-up, groups A-1, A-2, and A-3 exhibited similar flexion-extension ranges (122 ± 25°, 119° ± 18°, and 120° ± 16°, respectively) and modified Mayo wrist scores (87 ± 7, 89 ± 8, and 85 ± 9). Thus, the conservative and operative treatments provided similar long-term outcomes, and similar results were observed for patients with ulnar styloid process fracture at the tip or base. The average splint application period for patients who underwent conservative treatment was 6.6 weeks (range, 4-9 weeks). In our practice, conservative treatment (supination sugar-tong splinting) and surgical treatments provided similar long-term outcomes for acute DRUJ instability with distal radius fracture.