Individuals with congenital or acquired prolongation of the QT interval, or long QT syndrome (LQTS), are at risk of life-threatening ventricular arrhythmia^,. LQTS is commonly genetic in origin but can also be caused or exacerbated by environmental factors^,. A missense mutation in the L-type calcium channel Ca_V1.2 leads to LQTS in patients with Timothy syndrome^,. To explore the effect of the Timothy syndrome mutation on the electrical activity and contraction of human cardiomyocytes, we reprogrammed human skin cells from Timothy syndrome patients to generate induced pluripotent stem cells, and differentiated these cells into cardiomyocytes. Electrophysiological recording and calcium (Ca²⁺) imaging studies of these cells revealed irregular contraction, excess Ca²⁺ influx, prolonged action potentials, irregular electrical activity and abnormal calcium transients in ventricular-like cells. We found that roscovitine, a compound that increases the voltage-dependent inactivation of Ca_V1.2 (refs ), restored the electrical and Ca²⁺ signalling properties of cardiomyocytes from Timothy syndrome patients. This study provides new opportunities for studying the molecular and cellular mechanisms of cardiac arrhythmias in humans, and provides a robust assay for developing new drugs to treat these diseases.