Autofluorescence (AF) bronchoscopy is a useful tool for early cancer detection. However the mechanisms involved in this diagnosis procedure are poorly understood. We present an in vivo autofluorescence imaging study to access the depth of the principal contrast mechanisms within the bronchial tissue comparing a narrow band and broad band violet fluorescence excitation. Knowledge of this parameter is crucial for the optimization of the spectral and optical design of clinical diagnostic AF imaging devices. We observed no differences in the chromatic contrast using the two excitation modes, indicating that the principal contrast mechanisms have a non-superficial character. © 2005 SPIE-OSA.