Induced pluripotent stem cells (iPSCs) have shown promise in investigating donor-specific phenotypes and pathologies. The iPSC-derived cardiomyocytes (iPSC-CMs) could potentially be utilized in personalized cardiotoxicity studies, assessing individual proarrhythmic risk. However, it is unclear how closely iPSC-CMs derived from healthy subjects can recapitulate a range of responses to drugs. It is well known that QT-prolonging drugs induce subject-specific clinical response and that all healthy subjects do not necessarily develop arrhythmias or exhibit similar amounts of QT prolongation. We previously reported this variability in a study of four human ether-a-go-go-related gene (hERG) potassium channel-blocking drugs in which each subject underwent intensive pharmacokinetic and pharmacodynamic sampling such that subjects had 15 time-matched plasma drug concentration and electrocardiogram measurements throughout 24 hours after dosing in a phase I clinical research unit. In this study, iPSC-CMs were generated from those subjects. Their drug-concentration-dependent QT prolongation response from the clinic was compared with in vitro drug-concentration-dependent action potential duration (APD) prolongation response to the same two hERG-blocking drugs, dofetilide and moxifloxacin. Comparative results showed no significant correlation between the subject-specific APD response slopes and clinical QT response slopes to either moxifloxacin (P = 0.75) or dofetilide (P = 0.69). Similarly, no significant correlation was found between baseline QT and baseline APD measurements (P = 0.93). This result advances our current understanding of subject-specific iPSC-CMs and facilitates discussion into factors obscuring correlation and considerations for future studies of subject-specific phenotypes in iPSC-CMs.