A simple spectral computation method is introduced for the evaluation of differences in dummy-head Head-Related Transfer Functions (HRTFs). These transfer functions describe the directional-dependent filtering of the outer ears and are responsible for basic localization cues. Head-Related Transfer Function Differences (HRTFDs) are defined as the quotient of measured HRTFs in the same direction, comparing the cases of modified environmental conditions. The HRTFDs are relative measures and thus free from systematic errors of the measurement system and individual parameters. Our experience shows that HRTFDs are well suited to analyze the fine structure of measured HRTF data and for representation of the changes in the acoustical environment near or on the head. HRTFDs of a BK dummy-head will be shown representing differences from about 1 dB. This measurement accuracy was determined by replacing the dummy-head with an omnidirectional microphone.