Short-term maintenance of different stable steady state balances for protoporphyrin production, accumulation in erythrocytes and plasma, and fecal excretion was demonstrated in 5 patients with uncomplicated protoporphyria in whom red blood cell, plasma, and fecal protoporphyrin levels were determined on 5-7 sequential days. The patients were hospitalized in a research unit for dietary control and standardization of specimen collection. Each patient was found to quantitatively partition protoporphyrin among these compartments in a pattern that was reproducible from day to day, and characteristic for that patient. These observations partially substantiate a previously proposed hypothesis that protoporphyrin partitioning patterns may provide a criterion for prospective classification of these patients into groups that may ultimately be shown to have different risk for development of symptomatic liver disease.