The incidence of diarrheal illnesses on cruise ships has declined, but many of the outbreaks that still occur are preventable. Researchers used data from the Vessel Sanitation Program, which inspects cruise ships at US ports twice a year and reports information about disease outbreaks to the CDC. Between 1989 and 1993, there were 13,442 cruises between 3 and 15 days duration. During that time, there were 21 outbreaks of diarrheal illness on the ships, giving an incidence of 1.4 outbreaks per 1,000 cruises. Between 1986 and 1993, there were 31 outbreaks of diarrheal illness. Twenty-five occurred on cruises to the Caribbean or Mexico. The source of the illness was determined in 58% of the cases. Undercooked shellfish and eggs were the most common sources of contamination. In two outbreaks, a food handler was found to be transmitting the infection. The incidence of such outbreaks could be substantially reduced by cooking food properly, avoiding local caterers and excluding infected people from handling food.