Laser-induced fluorescence (LIF) can be used for noninvasive spectroscopic identification of biological tissue and is of special interest in early tumor detection. The basis for this optical biopsy method is the interaction of the laser light with tissue chromophores, such as tryptophan, collagen, elastin, NADH, beta-carotene and hemoglobin. The UV-excited fluorescence that arises from the native chromophores, the autofluorescence, has a broad distribution, peaking at about 490 nm with a lower intensity in tumor compared to normal tissue. The tumor detection potential is enhanced with exogenously administrated tumor- marking agents, such as hematoporphyrin (HPD, commercial name Photofrin), with two fluorescence peaks at about 630 and 690 nm. We have developed clinical instrumentation both for tissue point monitoring and for full real-time image processing. Seventy-one patients were investigated in vivo and surgical samples from additional 20 patients. In 46 patients the autofluorescence only was monitored. In 45 patients low-dose Photofrin injection was used. The in vivo investigations included different kinds of lung tumors, urinary bladder tumors, and malignant gliomas. The in vitro measurements were performed in breast tumors and prostatic tumors. Invasive and early tumors and also precancerous lesions can be revealed utilizing LIF in low-dose Photofrin injected patients.