Time‐matched baseline subtraction (TMBS) is one of the recommended methods to assess QT prolongation of drugs. This study evaluates the sensitivity/reliability of TMBS. Individual heart rate corrected QT (QTc) data were obtained from 22 healthy females who had baseline QT on two consecutive days with ≥9 readings over 24h on either day. 1000 re‐sample datasets were generated via randomly selecting individual data series from one of the 2 days as the basis for subtraction. For each dataset, the inter‐day difference in QTc at time‐matched points (ΔQTc) and its maximum (ΔQTc,max) were determined for all subjects. The mean ΔQTc,max for the 22 subjects and its distribution for the 1000 replicates were then computed. Individual QTc time profile showed fluctuations without a common pattern among all subjects or within a subject across the 2 days. No subject had a ΔQTc of >30ms (highest: 24 ms). The within‐day standard deviation (SD) for individual subjects ranged 2.5‐8.1 ms. Many subjects had even higher between‐day SD. For the 1000 replicates, the mean ΔQTc,max ranged 3.8‐13.3 ms and SD ranged 4.0–8.4 ms. This investigation raises a question on the sensitivity/reliability of TMBS method. Additional studies/populations will be similarly investigated. Clinical Pharmacology & Therapeutics (2004) 75, P56–P56; doi: