Objectives To minimise adverse events in healthcare, various large-scale incident reporting and learning systems have been developed worldwide. Nevertheless, learning from patient safety incidents is going slowly. Local, unit-based reporting systems can help to get faster and more detailed insight into unit-specific safety issues. The aim of our study was to gain insight into types and causes of patient safety incidents in hospital units and to explore differences between unit types. Design Prospective observational study. Setting 10 emergency medicine units, 10 internal medicine units and 10 general surgery units in 20 hospitals in the Netherlands participated. Patient safety incidents were reported by healthcare providers. Reports were analysed with root cause analysis. The results were compared between the 3 unit types. Results A total of 2028 incidents were reported in an average reporting period of 8weeks per unit. More than half had some consequences for patients, such as a prolonged hospital stay or longer waiting time, and a small number resulted in patient harm. Significant differences in incident types and causes were found between unit types. Emergency units reported more incidents related to collaboration, whereas surgical and internal medicine units reported more incidents related to medication use. The distribution of root causes of surgical and emergency medicine units showed more mutual similarities than those of internal medicine units. Conclusions Comparable incidents and causes have been found in all units, but there were also differences between units and unit types. Unit-based incident reporting gives specific information and therefore makes improvements easier. We conclude that unit-based incident reporting has an added value besides hospital-wide or national reporting systems that already exist in various countries.