Background and purpose - Epidemiological studies of full-thickness rotator cuff tears (FTRCTs) have mainly investigated degenerative lesions. We estimated the population-based incidence of acute FTRCT using a new diagnostic model. Patients and methods - During the period November 2010 through October 2012, we prospectively studied all patients aged 18-75 years with acute onset of pain after shoulder trauma, with limited active abduction, and with normal conventional radiographs. 259 consecutive patients met these inclusion criteria. The patients had a median age of 51 (18-75) years. 65% were males. The patients were divided into 3 groups according to the clinical findings: group I, suspected FTRCT; group II, other specific diagnoses; and group III, sprain. Semi-acute MRI was performed in all patients in group I and in patients in group III who did not recover functionally. Results - We identified 60 patients with FTRCTs. The estimated annual incidence of MRI-verified acute FTRCT was 16 (95% CI: 11-23) per 105 inhabitants for the population aged 18-75 years and 25 (CI: 18-36) per 105 inhabitants for the population aged 40-75 years. The prevalence of acute FTRCT in the study group was 60/259 (23%, CI: 18-28). The tears were usually large and affected more than 1 tendon in 36 of these 60 patients. The subscapularis was involved in 38 of the 60 patients. Interpretation - Acute FTRCTs are common shoulder injuries, especially in men. They are usually large and often involve the subscapularis tendon.