Previous studies assessing the treatment of port-wine stains (PWS) with the 585 nm pulsed dye laser have relied on either subjective clinical assessment or in vivo measurement of skin colour alone. The aim of the present retrospective study was to develop an objective method of assessing available pre- and post-treatment photograph pairs. Port-wine stains depicted in photographs of 23 patients following six or more treatment sessions were assessed for changes in colour (DeltaH*) and PWS size by computer image analysis and were compared with a subjective assessment of PWS reduction by a "blinded" physician examining the same images. The post-treatment mean reduction in the PWS assessed by the physician was 39.7%. A global assessment score incorporating values of DeltaH* and PWS size by computer analysis showed a mean reduction of 12%, with a more significant correlation with the physician assessment (Spearman's rank correlation coefficient 0.627; P=0.001) than changes in size or colour alone.