This paper uses matched employee-employer data from the British Workplace Employment Relations Survey (WERS) 2004 to examine the determinants of employee job anxiety and work-related psychological illness. Job anxiety is found to be strongly related to the demands of the job as measured by factors such as occupation, education and hours of work. Average levels of employee job anxiety, in turn, are positively associated with work-related psychological illness among the workforce as reported by managers. The paper goes on to consider the relationship between psychological illness and workplace performance as measured by absence, turnover and labour productivity. Work-related psychological illness is found to be negatively associated with several measures of workplace performance.