The porphyrins are precursors of heme, which is a cofactor in hemoglobin,myoglobin and the cytochromes. They play an important role in the oxygen metabolism. Heme biosynthetic activity is located quantitatively in bone marrow and liver. The series of reactions leading to heme synthesis begins with the condensation of succinyl coenzyme A (CoA) and glycine and ends with the insertion of an iron atom into a molecule of protoporhyrin IX. There are several genetic defects of heme biosynthesis described. In all of them an increase of porphyrins is recognized. The laboratory diagnosis and classification of porhyrin disorders are based primarily on the measurement of the excretion of porphyrins and porphyrin precursors, mainly in urine. Uro-, copro-, penta- und tricarboxyporphyrin are increased in autosomal dominant acute porphyrin disorders. In chronic porphyrin disorders, including Porphyria cutanea tarda, uro- and heptaporphyrin are elevated. In chronic lead intoxication the coproporhyrins are slightly increased (0.5-2 µmol/24 h urine), acute lead poisoning is characterized by extremly high excretion of total porphyrins (up to 15 µmol/24 h urine).