Occupation-related mental stress has been associated with significant loss in terms of diminished productivity, higher absenteeism, and increased workers' compensation claims. The Liberty Mutual Group workers' compensation data were analysed for the years 1984-93 for mental stress-related claims. This represented over 7 million claims, over 17000 of which were identified as mental stress-related. The proportion of all stress claims was estimated for each year. The proportion by gender, age and occupation (job classification code and occupation name) was also described. Stress claims increased during the late 1980s, peaking in 1991, accounting for 0.48% of all claims and 1.69% of all claims costs, and has declined since. Even at its peak, mental stress claims were not a major portion of workers' compensation losses. However, they are expensive. The average costs of a stress claim in 1993 was about $13000. The state of California accounts for 60% of the claims reported to Liberty Mutual over this 10-year period. In 1993 women accounted for 51% of the stress claims and about 30% of all claims. The mean age of workers with stress claims was 39.3 years, with most stress claims from 30 to 34 year-olds. High-risk occupations and industries include banks, insurance companies, general labourers, management, salespersons, and drivers. The current decline in stress claims can largely be explained by a combination of changes in unemployment, increasing litigation, and changes in law in California and other states that made more stringent the requirements whereby a mental stress claim can be considered work-related. While the data presented are helpful for comparing stress claims to all claims reported to Liberty Mutual and for identifying high-risk occupations and industries, because of the uniqueness of the stress claim selection algorithm and the uncertainties with cost estimates, the cost figures are not directly comparable to other claims reporting systems.