Background <p>Adult-onset still’s disease and septic arthritis share similar symptoms—fever, joint pain, and limited mobility—making postoperative diagnosis difficult after combined anterior cruciate ligament and posterior cruciate ligament reconstruction. In this case, through a series of examinations and diagnostic treatment, the final diagnosis was adult-onset still's disease.</p> Case presentation <p>A 41-year-old male developed recurrent fever and polyarthritis one week after undergoing arthroscopic combined anterior cruciate ligament and posterior cruciate ligament reconstruction. Clinical signs and tests were inconclusive. A multidisciplinary team was assembled to determine the diagnosis and establish a treatment plan. Despite broad-spectrum antibiotics, symptoms persisted. Typical adult-onset still's disease signs—rash, pharyngalgia, leukocytosis—emerged. Treatment with methylprednisolone, aspirin, and tocilizumab led to rapid improvement. Diagnosis was confirmed using Yamaguchi criteria. The patient was discharged in stable condition and remained symptom-free during follow-up. Knee function fully recovered.</p> Discussion and conclusion <p>This may be the first reported case of adult-onset still's disease following combined anterior cruciate ligament and posterior cruciate ligament reconstruction. In conjunction with the relevant literature, we summarize the differences in clinical symptoms between septic arthritis and adult-onset still's disease. Reviewing the patient's hospitalization process, we discuss the "controversial" diagnostic and therapeutic measures taken by the multidisciplinary team, along with any doubts and considerations. The case highlights the diagnostic challenge of differentiating adult-onset still's disease from postoperative joint infection.</p>