Administration of hematoporphyrin derivative (HPD) with the association of laser beam irradiation has recently been employed for the diagnosis and treatment of cancer. This methodology is based on the tumor affinity of HPD and the phenomenon of fluorescence generation under the specific wave laser beam. Furthermore, laser irradiation induces a photochemical reaction of HPD, which may kill the HPD uptake cancer cells. Utilizing the electron spin resonance (ESR) that produces HPD radicals, we measured the level of radicals in the lymphocytes separated from the peripheral blood of patients with cancer of various types and sites and that from normal controls, as well as in the culture cancer cells (K-562). The level of radicals in the lymphocytes from the cancer-bearing patients and culture cancer cells was found to be significantly elevated as compared with that in the lymphocytes of normal controls and patients with noncancerous conditions such as SLE, hepatic disease, etc. This procedure appears to be very useful for the diagnosis of cancer, and this is the first report in the world.