We conducted a retrospective study of incident reports concerning the national, nurse-led telephone triage system in Sweden. The Swedish Health Care Direct organization (SHD) is staffed by registered nurses who act as telenurses and triage the callers' need for care, using a computerized decision support system. Data were collected during 2007 from all county councils that participated in the SHD and were analysed using content analysis. Incident reports were then compared concerning differences in reported categories and who reported the errors. The 426 incident reports included 452 errors. Of the analysed incident reports, 41% concerned accessibility problems, 25% incorrect assessment, 15% routines/guidelines, 13% technical problems and 6% information and communication. The most frequent outgoing incident reports (i.e. sent from SHD to other health-care providers) concerned accessibility problems and the most frequently incoming reports (i.e. sent to SHD from other health-care providers) concerned incorrect assessment. There was a significant difference (P < 0.001) between outgoing and incoming reports regarding the main category. Telenurses have limited possibilities for referring the caller to their primary health-care provider or specialist, which may cause them to over-triage or under-triage the callers' need for care. This over-triage or under-triage may in turn cause other health-care providers to report incorrect assessment to SHD. The implications for practice are that poor accessibility is a matter that should be addressed and that the reasons for incorrect assessment should be explored.