The process of photodynamic therapy (PDT) and its application for the treatment of cancer has been reported for veterinary patients(l,2,3,4,5,6). Multiple factors function together in the patient to influence the overall effectiveness of a PDT treatment, including tumor size, location and biology, efficiency of the photosensitizer to produce a cytotoxic reaction, systemic and tumor pharmacokinetics of the photosensitizer, tissue oxygenation, and light dosimetry, All of these factors need to be considered in evaluating a patient for possible PDT treatment, reviewing the design and results of any PDT clinical trial, and in proposing alterations to the approach of the clinical application of PDT where improvement of results is desired. The use of PDT has been reported several times for the treatment of facial, solar-induced squamous cell carcinoma (SCC) in cats. Several of these studies have employed the use of chloro-aluminum sulfonated phthalocyanine (AlPcS) as a photosensitizer, with varying degrees of clinical response, which the authors in each case felt were encouraging for the employment of PDT for this condition in cats. ( 4,5,6) From our clinical experiences in employing AlPcS for this condition in cats and in comparing our experiences to those of others, we are attempting to better understand this disease and the factors which could influence the effectiveness of PDT for its treatment.