Villarejos, V. M. (LSU-ICMRT, Apdo. 10155, San José, Costa Rica), D. A. Gutiérrez and W. Pelon. Identification of a type B hepatitis epidemic in Costa Rica. Comparative analysis of two outbreaks of viral hepatitis. Am J Epidemiol 96: 372–378, 1972.—A high percentage (67%) of hepatitis B antigen (HBA) positives was recently detected in a sample of sera from a 1963–1964 epidemic in Costa Rica, originally reported as infectious hepatitis. The HBA findings, together with sharply distinctive epi-demiologic characteristics viewed retrospectively support re-identification of that epidemic as a type B outbreak, in the absence of a parenteral mechanism of transmission. A comparison is made of the main features of this outbreak with those of an epidemic of infectious hepatitis which occurred four years later among the same population.