The noninvasive thermometry method is based on the temperature dependence of the proton resonance frequency (PRF). High‐quality temperature images can be obtained from phase information of standard gradient‐echo sequences with an accuracy of 0.2°C in phantoms. This work was focused on the in vivo capabilities of this method. An experimental setup was designed that allows a qualitative in vivo verification. The lower‐leg muscles of a volunteer were cooled and afterwards reheated with an external water bolus. The temperature of the bolus water varied between 17°C and 37°C. The in vivo temperature images can be used to extract the temperature in muscle tissue. The data in the fat tissue are difficult to interpret because of the predominance of susceptibility effects. The results confirm the method's potential for hyperthermia control.