Rationale:To improve the diagnosis of life-threatening Bacilli Calmette Guerin (BCG) arterial aneurysm in patients treated by intravesical instillation of BCG vaccine as adjunctive therapy for non-muscular bladder carcinoma, is a life-threatening condition. Its diagnosis remains cumbersome.Patient concerns:One patient with a history of intravesical BCG installation presented with aortic aneurysm with routine microscopic examination after Ziehl-Neelsen staining remaining negative.Diagnoses:We used fluorescence in situ hybridization (FISH) to target the Mycobacterium tuberculosis complex rpob gene in a fresh aortic specimen. FISH yielded fluorescent mycobacteria in aortic lesions; mycobacteria were further confirmed as Mycobacterium bovis BCG mycobacteria by polymerase chain reaction (PCR) sequencing.Interventions:The patient benefited from an antituberculous treatment combining rifampicin, isoniazid, and ethambunol.Outcome:A 9-month follow-up indicated a favorable outcome.Lessons:This case report teaches that FISH targeting the M tuberculosis complex rpoB gene should be incorporated in the laboratory investigation of aortic aneurysm in patients with a history of bladder carcinoma.