A questionnaire survey of motion sickness occurrence on board passenger ferries has been conducted. Data were collected from 20,029 passengers on 114 voyages on 9 vessels: 6 ships, 2 hovercraft, and 1 jetfoil. Information was obtained about feelings of illness, the occurrence of vomiting, the taking of anti-seasickness tablets, the consumption of alcoholic drinks, regularity of travel by sea, age, and sex. Overall, 7% of passengers reported vomiting at some time during the journey, 21% said they felt "slightly unwell," 4% felt "quite ill," and a further 4% felt "absolutely dreadful." Both vomiting incidence and illness rating were greater in females than in males, and there was a slight decrease in sickness occurrence with increasing age. The incidence of vomiting was related to the taking of tablets and the drinking of alcohol; there were also some interaction effects with other variables. Anecdotal information from passengers is reported and consideration is given to the effects of environmental variables.