Background Diagnosis of short QT syndrome ( SQTS ) remains difficult in case of borderline QT values as often found in normal populations. Whether some shortening of refractory periods ( RP ) may help in differentiating SQTS from normal subjects is unknown. Methods and Results Atrial and right ventricular RP at the apex and right ventricular outflow tract as determined during standard electrophysiological study were compared between 16 SQTS patients ( QT c 324±24 ms) and 15 controls with similar clinical characteristics ( QT c 417±32 ms). Atrial RP were significantly shorter in SQTS compared with controls at 600‐ and 500‐ms basic cycle lengths. Baseline ventricular RP were significantly shorter in SQTS patients than in controls, both at the apex and right ventricular outflow tract and for any cycle length. Differences remained significant for RP of any subsequent extrastimulus at any cycle length and any pacing site. A cut‐off value of baseline RP <200 ms at the right ventricular outflow tract either at 600‐ or 500‐ms cycle length had a sensitivity of 86% and a specificity of 100% for the diagnosis of SQTS . Conclusions Patients with SQTS have shorter ventricular RP than controls, both at baseline during various cycle lengths and after premature extrastimuli. A cut‐off value of 200 ms at the right ventricular outflow tract during 600‐ and 500‐ms basic cycle length may help in detecting true SQTS from normal subjects with borderline QT values.