Aim. This paper is a report of a study conducted to determine the combined effect of acute and chronic job demands on acute job strains experienced during medical emergencies, and its consequences for individual teamwork behaviour. Background. Medical emergency personnel have to cope with high job demands, which may cause considerable work stress (i.e. job strains), particularly when both acute and chronic job demands are experienced to be high. This may interfere with effective individual teamwork behaviour. Methods. A cross-sectional survey study was conducted in 2008, involving 48 members (doctors and nurses) of medical emergency teams working in the emergency department of a Dutch general hospital. Data were analyzed by means of hierarchical regression analyses. Results. High acute job demands impeded effective teamwork behaviour, but only when they resulted in acute job strain. Acute emotional demands were more likely to result in acute job strain when chronic emotional job demands were also experienced as high. Although acute cognitive and physical strains were also detrimental, effective teamwork behaviour was particularly impeded by acute emotional strain. Conclusion. Acute job strains impair effective individual teamwork behaviour during medical emergencies, and there is urgent need to prevent or reduce a build-up of job strain from high acute and chronic demands, particularly of the emotional kind.