Summary Red light (c. 630 nm) is almost universally used in tumour phototherapy as it is the most penetrating of the porphyrin excitation wavebands. However, measurements of tumour attenuation of light of different wavelengths and of the excitation spectrum of haematoporphyrin derivative in vitro suggested that green light might be more efficient than red in destroying thin tumours. Experimentally, we confirmed this for tumours up to -1.2mm thick, a depth exceeding that of most carcinomas-in-situ. The superiority of green light over red in terms of the illumination time required to produce equivalent depths of necrosis may extend to greater depths (3-4 mm) if the former is produced by an argon laser and the latter by an argon-pumped dye laser.The relation between depth of necrosis zn and light dose D is shown to be zn=a x 'n(D/lA) where 0A iS the attenuation coefficient for light at wavelength A and 0A the threshold light dose for producing necrosis at that wavelength. This logarithmic relationship suggests that it may be difficult to eradicate large tumours merely by increasing the light dose, and indicates the need for other approaches.