Cases:This case report describes 3 pediatric patients presenting with acute calf or knee pain, calf swelling, and a ruptured popliteal cyst diagnosed by magnetic resonance imaging. Lyme disease was serologically confirmed in each case. In all instances, treatment was delayed because of atypical presentation. All patients responded favorably after antibiotic therapy.Conclusion:The differential diagnosis of Lyme disease should be considered in the context of children presenting with atraumatic unilateral calf pain and a ruptured popliteal cyst. Otherwise, this unusual presentation could delay diagnosis or result in unnecessary surgical intervention, particularly in pediatric patients.