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Background: The voltage-gated potassium channel Kv7.1 encoded by KCNQ1 is located in both cardiac myocytes and insulin producing beta cells. Loss-of-function mutations in KCNQ1 causes long QT syndrome along with glucosestimulated hyperinsulinemia, increased C-peptide and postprandial hypoglycemia. The KCNE1 protein modulates Kv7.1 in cardiac myocytes, but is not expressed in beta cells. Gain-of-function mutations in KCNQ1 and KCNE1 shorten the action potential duration in cardiac myocytes, but their effect on beta cells and insulin secretion is unknown. Case presentation: Two patients with atrial fibrillation due to gain-of-function mutations in KCNQ1 (R670K) and KCNE1 (G60D) were BMI-, age-, and sex-matched to six control participants and underwent a 6-h oral glucose tolerance test (OGTT). During the OGTT, the KCNQ1 gain-of-function mutation carrier had 86% lower C-peptide response after glucose stimulation compared with matched control participants (iAUC 360min = 34 pmol/l*min VS iAUC 360min = 246 ± 71 pmol/ l*min). The KCNE1 gain-of-function mutation carrier had normal C-peptide levels. Conclusions: This case story presents a patient with a gain-of-function mutation KCNQ1 R670K with low glucosestimulated C-peptide secretion, additionally suggesting involvement of the voltage-gated potassium channel KCNQ1 in glucose-stimulated insulin regulation.