Three methods of measuring the surface area of the involved skin were compared in 10 patients with psoriasis. Using the rule of nines, four untrained observers estimated the average extent of psoriasis as 20, 14, 23 and 33% of the body surface area. Measuring the area of tracings of plaque outlines using image analysis gave a mean involved surface area of 9% and image analysis of whole body photographs gave a mean value of 7%. We conclude that untrained observers using the rule of nines will overestimate the extent of psoriasis and that image analysis of whole body photographs is comparable to that of traced outlines.