Ulnar impaction syndrome (UIS) is a common source of ulnar-sided wrist pain, yet not all cases of radiographic ulnar impaction are symptomatic. We retrospectively analyze clinical or radiologic factors that affect prognoses of conservative treatment for idiopathic UIS. A total of 114 patients who had been diagnosed with UIS were treated with 6 weeks of short arm orthosis followed by formal physiotherapy for 6 weeks, with lifestyle modification to limit aggravating movements. The response to treatment, including pain numeric rating scale on an ulnar provocation test, grip strength, Disability of the Arm, Shoulder, and Hand score (DASH), was assessed at 24-week follow-up. For the 24-week follow-up, 29 patients (25%) underwent ulnar shortening osteotomy due to persistent symptoms after conservative treatment, and 18 (16%) patients had pain scores of greater than 5, but they had not undergone surgery. After controlling for confounding variables, female gender (odds ratio (OR) 1.39), duration of symptom (OR 1.27), high pain NRS score on provocation test (OR 1.45), and enhanced carpal or distal ulna bone on MRI (OR 1.82) were associated with a higher likelihood of treatment failure. Knowledge of the factors offers physicians insight into predicting prognoses and helps patients set realistic expectations.