Fifty-seven patients with variegate porphyria belonging to nine families are described. The prevalence of variegate porphyria in Finland is 1.3 per 100 000. Eighteen patients had had acute attacks. During on average 5.7 years' follow-up only two of 48 patients had symptoms, which were temporary and acute. The length of life of 13 genealogically traced gene carriers who lived mostly during the 19th century did not differ from that of the general population. Skin fragility occurred in 45 per cent of the patients and was usually mild. No patients were sensitive to sunlight but seven of 14 tested reacted abnormally to artificial light. The characteristic increase in the excretion of faecal proto- and coproporphyrin was usually apparent even in the latent stage of porphyria, but increased excretion of urinary porphyrin precursors occurred only during an acute attack. However, in seven of 48 patients studied during the latent stage, faecal prophyrins were only slightly elevated or even normal, causing problems in diagnosis. Determination of faecal X-porphyrin was of no help in diagnosis. Red cell free protoporphyrin was significantly higher and serum haemopexin lower than in controls. Red cell uroporphyrinogen I synthase activity was normal in 17 of 18 patients studied. We conclude that variegate porphyria may be commoner than hiherto suspected; only half the patients in temperaure and cold climates have skin symptoms, the risk of developing an acute attack is low, and some adult patients excrete normal or only slightly abnormal amounts of porphyrin.