In temperate and cold climates the most usual presenting symptom of variegate porphyria is an acute porphyric attack, indistinguishable from that seen in acute intermittent porphyria. Increased fragility of the skin in sun-exposed areas occurs in only half of such patients, and even then is usually mild and easily overlooked. The diagnosis depends on fecal excretion of porphyrins, which is greatly increased in variegate porphyria and consists predominantly of protoporphyrin. Urinary excretion of porphobilinogen and delta-aminolevulinic acid increases only during acute attacks. There are reasons for thinking that variegate porphyria is commoner than hitherto supposed. During an acute attack a patient without skin symptoms may well be misdiagnosed as having acute intermittent porphyria, because of identical symptoms and excretion of porphobilinogen in the urine. Thus, for a correct diagnosis, every patient presenting with symptoms of acute porphyria requires a fecal analysis.