During clinical trials of photodynamic therapy (PDT) the tissue fluorescence spectra under 510 nm laser excitation were recorded in 22 patients with tumors of the lungs, larynx, skin, gastric and esophageal carcinoma, and cancer of the gynecological organs before (autofluorescence) and after drug administration. A spectroscopic fluorescence detection system, developed at the General Physics Institute, Russian Academy of Sciences, was applied for tumor diagnostics. The system has been used for clinical fluorescence diagnostics with the aid of HPD-type photosensitizer Photogem (Moscow Institute of High Chemical Technologies) marked by selective retention in malignant tumors. Methods and equipment for spectra analysis aimed at estimating drug distribution in the near-surface layers of tissues, determining the tumor boundaries before photodynamic treatment, as well as studying drug pharmaco kinetics have been developed and tested. The results demonstrate the possibility of tumor detection by fluorescence even in low selectivity of drug accumulation which appears to be dependent on the stage and type of the disease and the organ involved.