This paper presents the background, design and baseline results of the Intervention Project on Absence and Well-being (IPAW). IPAW is a 5-year psychosocial work environment intervention study including 22 intervention and 30 control work-sites from three different types of work-site (a large pharmaceutical company, municipal technical services, and municipal nursing homes) in Copenhagen, Denmark. The baseline survey reported in this paper was conducted in 1996-97, and the cohort was followed until 2002. Interventions took place during 1996-98 at the organizational and interpersonal level and focused on psychological demands, social support, control, meaning of work, and predictability. The main end-points are self-rated health, perceived stress, absence from work, job satisfaction, and labour turnover. Analyses of the baseline data show good reliability of the psychosocial scales and a number of clear associations between psychosocial work environment factors and health indicators. The baseline data also demonstrate several discrepancies between the planned design of the study and the actual implementation of the project in practice.