Hemicrania continua is an uncommon primary headache disorder, characterized by continuous unilateral pain, where pain exacerbations are associated with cranial autonomic features. The hallmark of this condition is the absolute response to indometacin. We describe the phenotype of this condition in a large series of patients. Thirty-six (92%) patients had side-locked pain and 3 (8%) had side-alternating pain. The majority (82%) of the patients had the chronic (unremitting) form and the severity range of background pain was 1-10 out of 10 on verbal rating scale, with a mean of 6.5. Thirty-eight (97%) of the patients rated the painful exacerbations between 6.5 and 10 with a mean of 9 and 28 (71%) described their severe pain as excruciating. Of the cohort, 97% had at least one cranial autonomic feature during exacerbations: 73% had lacrimation, 51% nasal congestion, 46% conjunctival injection and 40% ptosis and facial flushing. Other cranial autonomic features included rhinorrhoea, forehead/facial sweating, itching eye, eyelid oedema, sense of aural fullness and periaural swelling, miosis, mydriasis and swelling of the cheek and face. Thirty-one (79%) had phonophobia, which was unilateral in 14 (48%); 29 (74%) had photophobia, which was unilateral in 14 (48%); and 27 (69%) had motion sensitivity. In addition, about two-thirds were agitated or restless, or both, and about one-quarter were aggressive, mainly verbally, with severe pain. All patients had a positive placebo-controlled indometacin test (100-200 mg intramuscularly) or a positive oral indometacin trial, or both. We suggest the International Headache Society criteria be revised to remove the absence of side-shift pain as a criterion. Furthermore, revised criteria should encompass a more extensive range of cranial autonomic features and consider pain as fluctuating with moderate, severe and very severe intensity. Currently the "sine qua non" for hemicrania continua is a response to indometacin. Since there is no reliable clinical marker of that response, we recommend an indometacin test, either orally or by injection, for any patient with unilateral pain, with or without cranial autonomic symptoms.