A method for the evaluation of regional lung ventilation using 81mKr eluted from a rubidium generator is described. The tracer distribution at equilibrium is a function of regional ventilation, not of volumes. The study can be performed on a wide range of patients, including unconscious and mechanically ventilated patients, and can be performed immediately following or concurrently with a perfusion study. Thus, precisely comparable ventilation and perfusion images can be obtained.