Neopterin is an intermediate purine metabolite which can be determined in serum and has been shown to be elevated in processes that stimulate cell mediated immunity. Neopterin serum levels have been measured in 37 patients diagnosed of boutonneuse mediterranean fever. Serum titers were higher during the acute phase than during the convalescence in 97.2% of patients. Neopterin levels normalized as the disease improved clinically. On the other hand, the highest levels were found in patients with a torpid evolution and appearance of complications.