Problems arising during application of a method proposed recently to predict tissue blood flow from measured thermal clearance curves have been discussed here. In this method, the treated tissue is divided into temperature equivalent zones with the aim to improve the mathematical description of experimental washout curves compared to descriptions based on monoexponential temperature decay. However, when applying the mathematical procedure suggested, any blood flow can be calculated by variation of the number of isothermal zones chosen. Unfortunately, to date no method for determination of the proper division of the treated tissue into temperature equivalent zones is available. Thus, any choice of the number of temperature equivalent zones is arbitrary. Therefore, we suggest that other investigators should not follow the analysis recently proposed for the determination of blood flow from thermal washout curves.