Noroviruses are the most common cause of viral gastroenteritis in the United States. To determine the magnitude and duration of virus shedding in feces, we evaluated persons who had been experimentally infected with Norwalk virus. Of 16 persons, clinical gastroenteritis (watery diarrhea and/or vomiting) developed in 11; symptomatic illness lasted 1-2 days. Virus shedding was first detected by reverse transcription-PCR (RT-PCR) 18 hours after participant inoculation and lasted a median of 28 days after inoculation (range 13-56 days). The median peak amount of virus shedding was 95 x 10(9) (range 0.5-1,640 x 10(9)) genomic copies/g feces as measured by quantitative RT-PCR. Virus shedding was first detected by antigen ELISA approximately 33 hours (median 42 hours) after inoculation and lasted 10 days (median 7 days) after inoculation. Understanding of the relevance of prolonged fecal norovirus excretion must await the development of sensitive methods to measure virus infectivity.