Over a 2-year period more than 1350 treatment sessions have been conducted with yellow 578 nm light from a copper vapour laser on a variety of vascular malformations. Of these sessions half were of port-wine stains, from 297 patients. The light is applied by scanning a 1 mm optical fibre approximately 2 mm above the lesion along the lines of Langer. We use a maximum scan rate of 3 s/cm2, which is the highest rate at which minimal blanching can be produced. Up to 6.5 watts of light has been used, and a typical energy fluence is 20 to 30 J/cm2. Topical melanin suppressing creams are used before and after treatment, in order to maximise the effectiveness of the treatment. Non-vasoconstricting anaesthetics are always used. Repeat treatments are 3 months apart. The scan rate we used was directly correlated with the light intensity. The degree to which the colour of the port-wine stains lightened after the first treatment was also well correlated with the scan rate (p<0.0005). The faster the scan rate, the greater the colour reduction. This provides some supporting evidence for the short illumination time that theoretical models have indicated as "ideal". The lightening was more marked for patients whose lesion colour was initially dark (p<0.0005). Those whose skin had a high melanin content (Maori, Polynesian and Asian) responded more slowly than others. Thirty-six patients have had a greater than 70% reduction in the size of their lesions after an average of four treatments. Ten patients have responded poorly after at least four treatments (10% of all those who have had four or more treatments). The occurrence of scarring, hyperpigmentation and hypopigmentation was low in comparison with that reported previously from the green light of argon ion lasers. © 1990 The Trustees of British Association of Plastic Surgeons. All rights reserved.