In Costa Rica, an annual outbreak of infant diarrheal disease (December and January) was reported since 1976, and rotavirus was incriminated later as the main etiological agent (1976-1981). Apparently the disease has not been systematically studied in Costa Rica after 1981. For that reason the occurrence of the outbreak was retrospectively documented for 1993-1995 and etiology was studied in 48 children treated for diarrhea at the Nacional Children Hospital (capital city of San Jose) during December, 1994 and January, 1995. Rotavirus (33%) and coronavirus (27%) were the main agents. To our knowledge, this is the first time that these viruses are incriminated in an outbreak of diarrhea.