Objectives To investigate the prevalence of arm-involvement in Klippel-Trénaunay syndrome (KTS)-patients and to describe the venous anatomy and/or venous aberrations present in the arm, and if possible, their relationship to complaints (pain, congestion and thromboembolic events). Methods A retrospective cohort-study was performed with data from medical records of a large KTS-cohort (n = 173) from a tertiary referral center. Within this cohort, a descriptive study (n = 12) was performed on the KTS-patients with arm involvement and who had been examined with Colour Duplex Ultrasonography (CDU). Results Our KTS-cohort (n = 173) comprised 43 patients (24.9%) with arm-involvement; in nineteen patients (11.0%) the arm was the only affected limb. Of those KTS patients investigated with CDU, 9 out of 12 (75%) had an aberrant venous anatomy. Conclusion Future research needs to clarity whether the complaints of KTS-patients in general are caused by an aberrant venous anatomy, coagulation alterations and/or other factors. Herein, the KTS-arms may play an important role.