Functional requirements of critical care information systems (CCIS) from the users' perspective.

•Critical care physicians and nurses consider CCIS as useful for their clinical work.•CCIS are considered useful even though the quality of available CCIS is rated low.•CCIS are considered useful by staff using CCIS and staff not using CCIS.•We specify 33 functions for critical care physicians that are considered as useful.•We specify 28 functions for critical care nurses that are considered as useful.