Summary.The purpose of the study was to compare the flare‐up rate for single‐visit endodontics among teeth without radiographic or clinical signs of apical periodontitis, those with radiographic or clinical signs of apical periodontitis not previously root‐treated, and those with apical periodontitis where retreatment was performed.All teeth were instrumented to a predetermined minimum size with a 0.5 per cent solution of sodium hypochlorite being used as the irrigant. The root canal was obturated without regard to the presence or absence of symptoms or diagnosis of the apical condition. The patients were given written postoperative instructions and a prescription for 600 mg ibuprofen to be taken if mild to moderate pain developed. If severe pain and/or swelling developed, the patient was instructed to telephone immediately and was considered to have had a flare‐up.Teeth without signs of apical periodontitis did not have any flare‐ups. One flare‐up occurred in 69 teeth with signs of apical periodontitis not previously root‐treated. The majority of the flare‐ups (3 of 22 teeth) occurred in teeth with signs of apical periodontitis requir