Forty-three patients with metabolic syndrome (MS) were examined. The urinary (uroporphyrin--UP and coproporphyrin--CP) and fecal (CP and protoporphyrin) fractions of porphyrin, as well as the urinary excretion of porphyrin precursors (S-aminolevulinic acid and porphobilinogen) were measured. Porphyrin metabolic disturbances were registered in 33 (76.7%) patients. Nine of these patients displayed such qualitative changes as fraction mismatch (CP/UP < 1; the normal value is 2.1 +/- 0.4), and an increase in the level of porphyrin precursors, while their total urinary porphyrin level was normal. In 24 patients pathological changes in porphyrin exchange were characterized by such quantitative changes as a many-fold increase in urinary and/or fecal porphyrin fraction as well as the development of secondary biochemical coproporphyrinuria syndromes, symptomatic elevation of fecal porphyrin level, and latent late cutaneous porphyria. Changes in porphyrin exchange in patients with metabolic syndrome broaden the scope of disturbances occurring in this syndrome, and allow considering these changes as additional criteria.