BackgroundSymptomatic osteoarthritis poses a major challenge to primary health care but no studies have related accessing primary care (‘detection’), receiving recommended treatments (‘treatment’), and achieving adequate control (‘control’). ObjectiveTo provide estimates of detection, treatment, and control within a single population adapting the approach used to determine a Rule of Halves for other long-term conditions. SettingGeneral population. Participants400 adults aged 50+ years with prevalent symptomatic knee osteoarthritis. DesignProspective cohort with baseline questionnaire, clinical assessment, and plain radiographs, and questionnaire follow-up at 18 and 36 months and linkage to primary care medical records. Outcome measures‘Detection’ was defined as at least one musculoskeletal knee-related GP consultation between baseline and 36 months. ‘Treatment’ was self-reported use of at least one recommended treatment or physiotherapy/hospital specialist referral for their knee problem at all three measurement points. Pain was ‘controlled’ if characteristic pain intensity <5 out of 10 on at least two occasions. ResultsIn 221 cases (55.3%; 95%CI: 50.4, 60.1) there was evidence that the current problem had been detected in general practice. Of those detected, 164 (74.2% (68.4, 80.0)) were receiving one or more of the recommended treatments at all three measurement points. Of those detected and treated, 45 (27.4% (20.5, 34.3)) had symptoms under control on at least two occasions. Using narrower definitions resulted in substantially lower estimates. ConclusionOsteoarthritis care does not conform to a Rule of Halves. Symptom control is low among those accessing health care and receiving treatment.