The photoxicity of coal tars was determined by comparing the ultraviolet light (UVL) energy required to produce erythema at tar treated sites (minimal phototoxic dose [MPD]) with the energy required to produce the same degree of erythema at untreated control sites (minimal erythema dose [MED]). The ratio of MED/MPD is the photoxic index (PI). Tars that were phototoxic had a PI of greater than 1. Using a UVA (320 to 400 nm) and a tuvb (290 to 320 nm) light source, 15 subjects and six tars were tested. All tars were phototoxic to UVA but not to UVB (P smaller than 0.0001). Although tar and UVL is a widely accepted treatment for psoriasis (Goeckerman therapy), the light sources employed at normal exposure times provide insufficient UVA energy to produce a phototoxic reaction to the tars are used. The therapeutic response seen in psoriatic patients treated with tar and UVL should therefore not be attributed to tar phototoxicity.