CDC’s Vessel Sanitation Program (VSP) monitors cases of acute gastroenteritis (AGE) on board cruise ships traveling to a U.S. port (1). Persons who have ≥3 loose stools (or more than normal for that person) within a 24-hour period or vomiting plus one other sign or symptom (e.g., fever, diarrhea, bloody stool, myalgia, abdominal cramps, or headache) meet the case definition for reportable AGE (2). When the percentage of passengers or crew members with AGE is ≥2% and the ship is due to arrive at a U.S. port within 15 days, the Maritime Illness Disease Reporting System alerts VSP and activates an investigation (1). During the first week of January 2023, VSP was notified of cases of AGE affecting >2% of passengers on board a ship that had completed three voyages in Europe and was within 15 days of arriving at a U.S. port (voyage 4)* (Figure). Ship medical crew members submitted stool samples from ill travelers for testing. All samples tested positive for norovirus genotype II. While the ship was sailing to a U.S. port, VSP monitored AGE cases on board and reviewed case data. By mid-January, passenger AGE prevalence reached 3.4%.