For objective evaluation of the treatment of psoriasis vulgaris standard techniques are desirable. They should be reproducible, sensitive and non-invasive. In this study non-invasive bioengineering techniques, especially high frequency/high resolution ultrasound for measurement of the healing of psoriasis vulgaris were evaluated. Fifty patients with chronic stationary plaque type psoriasis participated in a prospective study; in each patient two psoriatic plaques were examined by means of sonography, colorimetry and image analysis during treatment until complete resolution had occurred. Skin thickness and density could be quantified by means of high frequency ultrasound. In active psoriatic lesions, an echopoor area underneath the entry echo in the ultrasound image caused by acanthosis and inflammatory infiltrate is typical. Under therapy the thickness of this echopoor area diminishes while its density increases. Intensity of the erythema especially the decrease of erythema through healing could not exactly be quantified with the colorimeter because the 'Lab'-CIE-colour representation system cannot distinguish well enough between the colours red and brown. Image analysis allowed to measure the sizes of the psoriatic plaques and to quantify their resolution under therapy. The measuring of plaque size by the aid of computer based image analysis is possible and useful.