Human parvovirus B19 has been associated with various clinical effects in a number of uncontrolled reports. To define the usual manifestations of B19 infection in adults and the factors that influence them we present a clinicoepidemiological study of an outbreak of B19 infection centered on a junior school. Four hundred fifty-three of 475 adults in this community were interviewed and blood was obtained for serological diagnosis. Fifty-four cases of recent infection were identified and were HLA typed. Fourteen of the cases were asymptomatic; 32 had an influenzalike illness; 23 a rash; and 26 an acute-onset polyarthropathy that was more common in women and lasted for up to 7 months. HLA-A, -B, and -C antigen frequencies were similar to a local control population and showed no association with symptoms except that HLA-DR1 was absent in those with persistent arthropathy.