An outbreak of human illness caused by Mayaro (MAY) virus occurred in Belterra, Pará, Brazil in the first half of 1978. A total of 55 cases were confirmed, 43 by virus isolation and serology, and 12 by serology alone. The disease in Belterra presented as a distinct clinical syndrome characterized by fever, arthralgia and exanthema. No fatalities could be attributed to MAY virus infection. Arthralgia, accompanied by joint edema in 20% of cases, was a very prominent sign which caused temporary incapacity in many patients. Arthralgia was present in virtually all confirmed cases and persisted in some for at least 2 months, although with decreasing severity. Rash was present in two-thirds of the cases, and was either maculopapular or micropapular. The incidence of rash was higher in children than in adults. Contrary to arthralgia, which started with the onset of clinical illness, rash usually appeared on the 5th day and faded within 3-4 days. Fever, chills, headache, myalgia, lymph-adenopathy and other minor clinical manifestations were also recorded, and generally persisted for from 2-5 days. Leucopenia was a constant finding in all cases. Mild albuminuria was seen in four of 25 patients, and slight thrombocytopenia was seen in 10 of 20 cases. The fact that viremia levels higher tha 5.0 log10/1.0 ml of blood were recorded in 10 patients rises the possibility that man may have be an amplifying host in the MAY virus cycle. The MAY virus illness, as seen in Belterra, has clinical features similar to those observed in persons infected with chikungunya virus.