BACKGROUND AND OBJECTIVES: One principal mechanism of photodynamic therapy (PDT) in tumors is destruction of tumor-associated vasculature. In the present study, the vascular effects of PDT in tumors were investigated with power Doppler ultrasonography. MATERIALS AND METHODS: Seven cutaneous squamous cell carcinomas in cats were treated. Tumors were examined via power Doppler ultrasonography before, 5 minutes, 1 hour, and 24 hours after PDT. Images were digitized for computer-aided assessment of vascularity and blood volume. RESULTS: Mean baseline tumor vascularity and blood volume were moderate. During PDT, a significant decrease in vascularity and blood volume was noted. Lowest values were found 24 hours after PDT. CONCLUSIONS: Power Doppler ultrasonography represents a non-invasive modality to successfully monitor the vascular effects and thus, treatment efficacy, of PDT.