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The autofluorescence spectra of human tissues excited with the 365-nm line of pulsed xenon ion laser have been measured. The autofluorescence spectra of cancer tissues usually show characteristic peaks near 630 nm and 690 nm, which do not appear in the spectra of the corresponding normal tissues. This characteristic fluorescence can be taken as a criterion for cancer diagnosis; a consistency of 89% with the traditional biopsy method has been obtained in preliminary clinical application to the diagnosis of buccal carcinoma. Spectroscopic investigations suggest that the porphyrin compounds localized and retained in cancer tissues might be responsible for this characteristic autofluorescence.