Hendriks sa, sollie jw, nijnuis mg, stolper cf. Rare diseases: an impossible diagnostic challenge? huisarts wet 2016;59(11):498-501. Rare diseases are those with a prevalence of less than 1 in 2000 and about 8000 have been identified. While it is impossible for general practitioners (gps) to be able to diagnose all these diseases, it is important that they are alert to possible diagnoses and order investigations or refer the patient on as soon as possible. Gps have excellent diagnostic tools – awareness, recognition of an unusual combination of signs, ‘gut feelings’, knowledge of the family history, and ability to listen to the patient. The availability of reliable online resources can back up suspicions and contribute to a timely referral to secondary or tertiary care. Limiting the diagnostic delay is important, especially because cure is often not possible and patients can benefit from appropriate symptomatic treatment and access to facilities and services. Even if the exact diagnosis is not established, patients benefit from the support and supervision provided by their gp.