A significant clinical problem in the local treatment of cutaneous metastases of breast cancer (by any modality - surgery, radiation therapy or photodynamic therapy) is the fact that the disease almost always extends beyond the boundary of visible lesions in the form of microscopic deposits. These deposits may be distant from the site of visible disease but are often in close proximity to it and are manifested sooner or later by the development of recurrent lesions at the border of the treated area, thus the 'marginal miss' in radiation therapy, the 'rim recurrence' in photodynamic therapy, and the 'incisional recurrence' following surgical excision. More intelligent use of these treatment modalities demands the ability to detect microscopic deposits of tumor cells using non-invasive methodology. In vivo fluorescence measurements have been made possible by the development of an extremely sensitive fiber optic in vivo fluorescence photometer. The instrument has been used to verify that fluorescence correlated with injected porphyrin levels in various tissues. The delivery of light to excite and detect background fluorescence as well as photosensitizer fluorescence in tissues has been accomplished using two HeNe lasers emitting at 632.8 nm and 612 nm delivered through a single quartz fiber optic. Chopping at different frequencies, contributions of fluorescence may be separated. Fluorescence is picked up via a 400 micron quartz fiber optic positioned appropriately near the target tissue. Validation of these levels was made by extraction of the drug from the tissues with resultant quantitation. Recently, an extensive study was undertaken to determine if fluorescence could be used for the detection of occult, clinically non-palpable metastases in the lymph node of rats. This unique model allowed for the detection of micrometastases in lymph nodes using very low injected doses of the photosensitizer Photofrin II. Data obtained revealed the ability to detect on the order of 50-100 cells using 0.25 mg/kg of sensitizer, a level 20 times lower than normally used for treatment of animal tumors. These results indicate that Photofrin II could be used for fluorescence detection of small metastatic tumors, while substantially reducing the major side effect of PDT; namely, prolonged photosensitivity. Results to be presented demonstrate the ability of this technique to detect microscopic deposits of malignant tumor cells in patients with metastatic breast cancer. These deposits were found in clinically negative areas of the chest wall.