[A Cold Case Solved].

We present the case of a 43-year-old man who developed painless jaundice following a respiratory infection. Diagnostic workup revealed severe immune hemolytic anemia, specifically cold agglutinin syndrome (CAS), characterized by a positive direct antiglobulin test (DAT) for complement C3d and IgM antibodies. The patient's clinical course was complicated by significant hemolysis, requiring supportive care and multiple blood transfusions. Further microbiological investigation identified Mycoplasma pneumoniae as the underlying trigger for this secondary CAS.