Background: Studies of antipsoriatic therapy often rely on subjective scoring. Objective measures have been developed but have not previously been compared with subjective scoring. Objective: Our purpose was to compare subjective and objective measures of reduction of psoriasis with topical therapy. Methods: A 2-week, double-blind, left-to-right comparative trial of betamethasone valerate against white soft paraffin was performed in 12 patients. The subjective scores were erythema, elevation, scale, and a composite total. Objective measures were nitric oxide production measured by chemiluminescence; erythema reflectance; ultrasound scan for thickness, scale, and echo-poor zone; and computerized image analysis of video images. Results: Subjective and objective measures had similar power to detect therapeutic effect. The subjective measures showed greater variation and relatively overestimated improvement. There was correlation between measures and estimates for area, redness, and thickness. Nitric oxide production was the most powerful objective measure. Conclusion: Thickness determined by ultrasound scan and nitric oxide production are useful measures of reduction of psoriasis, which lend themselves to more powerful statistical tests than subjective interval data. (J Am Acad Dermatol 1997; 37:51–7.)