Background: Although the colors of psoriatic lesions, largely determined by erythema and scales, are important clinical indicators, expressing them in an objective manner is difficult. Cutaneous blood flow (CBF) also affects erythema. Serial measurement of these parameters during phototherapy was almost nonexistent. Objective: The objectives of our study were to observe the changes of color parameters and the CBF of psoriatic lesions during PUVA therapy and to determine their clinical significance. Methods: CBF, measured by laser Doppler flowmetry, and color parameters, measured by tristimulus colorimetry and reflectance spectrophotometry, were assessed in 13 patients with psoriasis who received PUVA therapy. Results: The values of CBF, erythema index (EI), and a^* (color parameter representing red-green axis) in psoriatic lesions were significantly different from those observed in the control sites before therapy. The parameters of psoriatic lesions normalized according to the clinical improvement and approached those of the control sites as PUVA therapy progressed. The values of melanin index (MI), L^* (color parameter representing white-black axis), and b^* (color parameter representing yellow-blue axis) showed no significant difference between the psoriatic plaques and the control sites. They all displayed changes toward darkening and indicated tanning induced by PUVA therapy. Serial changes presented a generally unidirectional pattern in the control sites. However, this was not always the case in psoriatic lesions because scale, infiltration, and erythema also affected the measurement of blood flow and the color of the skin. Conclusion: Color parameters and CBF were closely related with clinical improvement according to consecutive phototherapy. They may serve as objective indices for the visible morphology and underlying lesional pathophysiology of psoriasis. (J Am Acad Dermatol 2001;44:987-94.)