BACKGROUND Little has been reported on suicides among seafarers and how they have changed over time. OBJECTIVE To establish the causes, rates and trends in suicides at work among seafarers in UK merchant shipping from 1919 to 2005 and to compare suicide rates with the general UK population and with seafarers employed in non-UK shipping. METHODS Examination of seafarers' death inquiry files, death registers and death returns (for a total population of 11.90 million seafarer-years); literature reviews and national suicide statistics. RESULTS The suicide rate (for suicides at work and unexplained disappearances at sea) in UK shipping fell from 40-50 per 100,000 in the 1920s to <10 per 100 000 in recent years, with an interim peak during the 1960s. Suicide rates were higher for ratings (all ranks below officers) than for officers, for Lascars (Asian seafarers) than for British seafarers and for older than for younger seafarers and were typically lower than those in Asian and Scandinavian merchant fleets. The suicide rate (for suicides at work) among seafarers was substantially higher than the overall suicide rate in the general British population from 1919 to the 1970s, but following reductions in suicide mortality among seafarers, it has become more comparable since. CONCLUSIONS Although merchant seafaring was previously a high-risk occupation for suicides at work, there has been a sharp fall in the suicide rate in the past 40 years. Likely reasons for this include reductions over time in long intercontinental voyages and changes over time in seafarers' lifestyles.