A retrospective analysis of 130 patients was conducted in a Phase I oncology clinic to assess the effect of QTc formula selection on clinical trial eligibility. QTc values were calculated from screening electrocardiograms using 7 formulae (Bazett, Fridericia, Framingham, Hodges, Mayeda, Van de Water and Wohlfart). QTc values > 470 ms for females and > 450 ms for males were used to define prolongation. Concomitant medication potential for QTc prolongation was determined using a public database (AzCert). Ineligibility rates ranged from 3.1 % to 17.7 % (Framingham: 3.1 %, Van de Water: 3.1 %, Hodges: 3.1 %, Wohlfart: 3.1 %, Fridericia: 3.9 %, Bazett: 10.8 % and Mayeda: 17.7 %). A consistent ineligibility rate was achieved by using formulae-specific thresholds. Fifty one percent of patients were taking concomitant medications with QTc prolongation potential. The proportion of concomitant medications with the potential to prolong QTc was 11.57 % (96 of 830). Uniform criteria and guidelines for selection of QTc formulae need to be developed. Formulae-specific QTc thresholds also need to be specified.