Background The Sauvé-Kapandji procedure can result in instability of the proximal ulnar stump.Patients and methods We reviewed 44 patients (mean follow-up time 6 (0.6-13) years) to investigate predictive factors for ulnar instability after Sauvé-Kapandji operation. We used several scores including an instability score specifically designed for this study.Results Patients with a longer proximal ulnar stump had significantly lower instability scores, significantly better Mayo Modified wrist scores and DASH scores, and also less pain than those with shorter proximal ulna.Interpretation If the shortening of the proximal stump is less than 35 mm, a reliable improvement in motion and a high patient satisfaction can be expected. The risk of a painful ulnar instability is related to the amount of resection, and can be reduced by creating a long upper ulnar stump.