Objective: Nowadays, registration of patient data on paper is gradually being replaced by registration using an intensive care information system (ICIS). The aim of this study was to evaluate the effect of the use of an ICIS on nursing activity. Design: Randomized controlled trial with a crossover design. Setting: An 18-bed medical-surgical ICU in a teaching hospital. Patients, nurses and interventions: During a 6week period 145 consecutive adult patients admitted to the ICU after uncomplicated cardiothoracic surgery were randomized into two groups: for one group the documentation was carried out using a paper-based registration (Paper), in the second group an ICIS was used for documentation. Measurements and results: The nursing activities for these patients were studied during two separate periods: the admission period and the registration phase (the period directly following the admission procedure). The duration of the admission procedure was measured by time-motion analysis and the nursing activities in the registration phase were studied by work sampling methodology. All nursing activities during the registration phase were grouped in four main categories: patient care, documentation, unit-related and personal time. The duration of the admission procedure was longer in the ICIS group (18.1±4.1 versus 16.8±3.1 min, p<0.05). In the registration phase, a 30% reduction in documentation time (Paper 20.5% of total nursing time versus ICIS 14.4%, p<0.001), corresponding to 29 min (per 8h nursing shift) was achieved. This time was completely re-allocated to patient care. Conclusions: The use of the present ICIS in patients after cardiothoracic surgery alters nursing activity; it reduces the time for documentation and increases the time devoted to patient care. Electronic Supplementary Material: is available if you access this article at http://dx.org/10.1007/s00134-002-1542-9. On that page (frame on the left side), a link takes you directly to the supplementary material.