Background. Diagnostic error in primary care can have serious implications for the patient, the clinician and the health-care system, possibly more so than other types of error.|Objective. To identify common characteristics of diseases that GPs may misdiagnose.|Methods. Systematic search of the MEDLINE and EMBASE databases for primary research on diagnostic error/delay in primary care. Papers on system errors, patient delay, case reports, reviews, opinion pieces, studies not based on actual cases and studies not using a systematic sample were excluded from the review. Twenty-one papers were included. All papers were assessed for quality using the GRADE system. Common features were identified across diseases and presentations that made diagnosis difficult and led to error/delay.|Results. Most studies were retrospective cohorts of patients recruited in hospital and collected data from patient interviews and/or hospital records, resulting in incomplete and potentially biased information. It was usually not possible to determine preventability of the delay. Some conditions were extremely rare, suggesting a specialist research interest rather than an increased rate of misdiagnosis. Conditions investigated were malignancies, myocardial infarction, meningitis, dementia, iron deficiency anaemia, asthma, tremor in the elderly and HIV. Common features of difficulty were atypical presentations, non-specific presentations, very low prevalence, the presence of co-morbidity and perceptual features and could be missed.|Conclusions. Misdiagnosis in primary care covers a wide range of conditions that may be related in the manner in which they present. The challenge is to identify ways of supporting the diagnostic process in potentially difficult presentations.