Excess urinary porphyrin excretion, or porphyrinuria, results from inhibition of key enzymatic steps in such clinical conditions as genetic deficiencies in heme production enzymes, hepatitis, renal disease, and erythroid disease, as well as by heavy metal inhibition of heme enzyme synthesis. In both experimental animals and in humans exposed to heavy metals, elevated levels of porphyrins have been found in urine. Studies have been conducted using urinary porphyrin measurements as heavy metal (eg, mercury) biomarkers in Autistic Spectrum Disorders (ASD). Greater than 50% of the ASD patients were found to have significant elevations of urinary coproporphyrin (CP) and pentacarboxylporphyrin (5-CP) of more than twofold to threefold above the mean of controls. Significant reductions in 5-CP and CP were observed in ASD patients following chelation therapy. The authors concluded that mercury exposure may play a causal role in a significant number of ASDs and that additional research needs to be conducted to further evaluate mercury toxicity in ASDs.