Effective methods of recording and measuring progress of the signs of psoriasis are essential and central to the accurate assessment of therapy. Currently there is wide variation in assessment methods used, and even well-designed clinical assessment techniques have a wide observer error: objective assessment methods may have advantages. The clinical assessment techniques used in 30 clinical trials of treatment in chronic plaque psoriasis in 1985 and 1986 were reviewed and a detailed analysis is presented. In only 18 trials were individual regions assessed, and widely differing methods of scoring signs were used. Reproducibility of the assessment by clinicians of area of skin involvement by psoriasis was studied. Ten patients with chronic plaque psoriasis had standardized photography, and four clinicians assessed area of involvement from the photographs. The clinicians significantly differed in their rank ordering of the area of psoriatic skin involvement. Area of involvement by psoriasis in the same photographs was assessed objectively using computer-assisted planimetry. Although the clinicians' assessment did not differ randomly from the objective measurement, the accuracy of subjective area assessment is not the same for all clinicians. As even experienced clinical observers show wide variation when attempting to record clinical severity of psoriasis, there is clearly a need for better objective measures of psoriasis severity.