135 transmission spectra were obtained from 26 autopsy specimens of healthy as well as lipid atheromatous human abdominal aorta. The tissue samples were irradiated using a Nd:YAG laser (1064 nm, continuous wave, 60 W, 1 s) and analyzed by microspectrophotometry (spectral range 250 nm to 800 nm, cryosections 25 micrometers thick, area measured 16 micrometers in diameter). Spectra were recorded from native tissue and from the coagulation zone adjacent to the ablation area. The optical density of the coagulation zone was significantly increased compared to the untreated areas within all tissue samples over the entire spectral range investigated. In the ultraviolet increases were 2 to 6 fold for intima, 1.2 to 3 fold for media, 2 to 4 fold for adventitia and 1.2 to 2.5 fold for lipid atheromatous plaque respectively. In the visible spectral range increases were 7 to 12 fold for intima, 2 to 5 fold for media, 5 to 9 fold for adventitia and up to 3 fold for lipid atheromatous plaque respectively (data p < 0.001). In addition, differences in the optical density between lipid atheromatous plaque and normal vessel wall decreased after laser irradiation. This indicated that due to the changes of optical properties during laser irradiation data obtained from transmission spectra of the native vessel wall may not be sufficient to predict laser-tissue interaction at least with respect to continuous wave lasers.