Sweden has played a major role in the development of psychosocial work environment research. 1 In addition, Sweden, along with the other Nordic countries, has unique possibilities to link questionnaire data to administrative registers on demographics, employment and health. Despite this, the country has not previously had a prospective cohort with regularly repeated measures of work environment and health. The strengths of longitudinal studies in epidemiological research, as opposed to cross-sectional studies, are well known. Still, a significant part of the available evidence on associations between psychosocial work characteristics and health has been based on crosssectional studies, making it difficult to separate cause and effect. Both selection and reverse causation are often plausible alternatives to a causal interpretation. 2, 3 Associations may also be inflated by common method bias, since selfratings are often used for both exposures and outcomes. 4 Repeated measures of both psychosocial work factors and health outcomes have become more common, but are often treated with cross-sectional methodologies using information on exposure variables from one time point to predict outcome variables from the next. 5 Although preferable to cross-sectional studies, such approaches do not rule out reverse causation and contribute little or no understanding of causal mechanisms. Longitudinal studies with multiple repeat measures of both exposures and outcomes are therefore needed to advance our understanding of causality.

The need for a life course perspective on social causes of disease is also increasingly recognized. This includes the need to study the effects of accumulated exposures, differential effects in different phases of life (critical or sensitive periods), and chains of risk (where the disease outcome can be distal from the original social cause). 6, 7 Life course studies thus require cohorts that follow people during longer periods of their lives. This may be particularly salient in studies of labour market exposures, since earlier studies have often had an unstated and unrealistic assumption that work environment exposures are stable. The postindustrial labour market is in fact characterized by a relatively high degree of change, where individuals can expect to have many jobs, often in different occupations, across their working lives, possibly interspersed with spells of unemployment or further education. Internationally, there are several major longitudinal cohort studies with a focus on work environment and health. Some of them, such as the Whitehall II study in England, 8 the French GAZEL cohort 9, 10 and the Finnish Public sector study, 11 have multiple repeat measures on a range of factors concerning work, private life and health. All of these studies are, however, restricted to specific groups of employees: civil servants, employees at a gas or electricity company, or public sector employees, respectively. Existing nationally representative cohort studies, such as the prospective panels of the Danish Work Environment Cohort Study 12 and the Swedish Level of Living Survey, 13