An objective method is still lacking in the quantitative assessment of psoriasis severity. The purpose of this study was to examine whether computer image analysis (CIA) by a new colour segmentation method can be used as an objective method of estimating involved surface area in patients with psoriasis. Involved surface area in psoriasis was assessed from colour photographs covering the same areas as the psoriasis area and severity index (PASI) scores of 26 patients by human observers from different medical professional groups and by CIA. The reliability of CIA was tested with reference analysis by colouring manually the involved areas in the images with the aid of projected slides. The surface area estimates by the different methods were compared with each other and the resulting effects on the PASI score were assessed. The agreement between the reference analysis and the image analysis was high, especially on trunk areas, but the cylindrical shape of the limbs resulted in some difficulties in assessing involved skin area. The human eye estimates differed from the image analysis ones in almost one-third of the cases and mostly when psoriasis affected under 30% of the skin surface area. Both over-and underestimates emerged, but overestimates were more common. Error estimates had a significant effect on the PASI score. The CIA method seems to be reliable and practicable in estimating the actual surface of psoriasis. The method has the disadvantage of being time-consuming (photographing, processing of pictures) and technically demanding. Further development of this method should make it faster in the future.