During an epidemic of acute gastroenteritis in Helsinki, in March--May 1976, 18 out of 40 adult patients showed electron microscopic and/or serologic evidence for rotavirus infection. Rotavirus was most frequently seen in the fecal suspensions from 2 to 6 days after the onset of the symptoms but persisted in one patient for as long as 10 days. An increase in the complement-fixing (CF) serum antibody titers against the related Nebraska calf diarrhea virus (NCDV), or an initially high titer and subsequent significant decrease, was seen in all but one patient with rotavirus-positive feces, and in 6 additional patients. This suggests that using electron microscopy as the only diagnostic procedure a considerable number of rotavirus infections in adults remain undetected. Immune response against autologous or homologous rotavirus was also documented by immunoelectron microscopy. Complement-fixing antibody titers against NCDV decreased significantly from the convalescence values over a half-year observation period, but still remained clearly above the titers of a gastroenteritis-negative control population.