It is generally agreed that the incidence of motion sickness in sailors working below deck is higher than in sailors who have the horizon as a visual reference on the bridge. This study investigated the possible beneficial effect of a projected artificial horizon as a means to prevent seasickness. Twelve subjects were exposed to angular motion in a tilting room under three experimental conditions: a) With the windows covered, allowing no visual reference from the outside world; b) With the windows uncovered, thus allowing a partial view of the environment; and c) With the windows covered and a horizon projected on the walls by a rotating laser beam. Subjects were exposed for 35 min in each condition while performing different computerised tasks. There was a reduction in well-being and performance as a function of exposure time. These effects were clearly shown in the "closed cabin" condition. There were less motion sickness symptoms in the "artificial horizon" and "window" conditions. The presence of a visual reference prevented the decrement in performance found in the "closed cabin" condition. The results of this study suggest that a projected horizon might alleviate motion sickness aboard naval vessels, and thus improve the performance of sailors at sea.