Ten patients with variegate porphyria were uniformly found to have distinctive plasma porphyrin fluorescence wavelength maxima in saline-diluted plasma specimens. The porphyrin complex in each of these plasma samples had a fluorescence emission maximum at 626 +/- 1 nm. Twelve patients with porphyria cutanea tarda, eight patients with erythropoietic protoporphyria, one patient with congenital erythropoietic porphyria, two patients with acute intermittent porphyria, and four patients with hereditary coproporphyria, whose plasma specimens were similarly examined, had plasma fluorescence characteristics that were different from those of the patients with variegate porphyria. Plasma fluorescence emission that is maximal at 626 +/- 1 nm is a diagnostic marker for variegate porphyria.