Porphyrins and porphyrin precursors are normally detected in small amounts in healthy individuals but are found in large quantities in the urine, feces, blood, plasma, bone marrow, and liver in patients with various types of porphyrias. These are intermediates, or are derived from intermediates, in the pathway for heme biosynthesis. Heme is synthesized in all body tissues but in the largest amounts in the bone marrow and liver. Accurately measuring these compounds is important for diagnosis and monitoring of porphyrias. In addition, measurement of enzyme activities and mutation analyses by DNA sequencing enables confirmation of a porphyria diagnosis and genetic counseling. Biochemical approaches described here include measurements of porphyrin precursors and porphyrins in the urine, feces, plasma, erythrocytes, and liver, and determination of specific enzyme activities in erythrocytes and other cells. © 2025 Wiley Periodicals LLC.