A series of comparative exposures to both argon and krypton lasers have been made at 3 locations in a human retina--the fovea, the macula, and intraretinal vessels. In the fovea argon irradiations resulted in damage to both the inner and outer retinal layers as a result of absorption within the pigment epithelium and the macular pigment, while krypton exposures damaged the outer retina and the choroid. In the macula both systems resulted in damage to the outer retina, and again sufficient krypton radiation passed into the choroid to induce blood vessel occlusion, haemorrhage, and oedema. When intraretinal vessels were irradiated, only with argon was sufficient energy absorbed within the vessels to damage them or their surroundings in the inner retina. The implications of these findings are discussed in relation to the therapeutic uses of lasers.