We report on a 46-year-old woman, who developed streptococcus B septic polyarthritis in the context of septicemia of urinary origin. This case revealed a Good's syndrome whose diagnosis was made on the basis of a profound hypogammaglobulinaemia, a large decrease of peripheral B cells and a thymoma disclosed on chest computed tomography (CT) and confirmed by surgical removal (AB type). There was also an inversion of the peripheral CD4+/CD8+ T cell ratio and an increase of CD8+ T cells. The course of infection was favorable under treatment with antibiotics and intravenous immunoglobulin. Good's syndrome is a rare entity, which belongs to primary immunodeficiency syndromes. Its first manifestations appear late in life, usually after the age of 40. In front of hypogammaglobulinemia, it is necessary to search for Good's syndrome by practicing chest CT scan, looking for thymoma, which is constant, and peripheral lymphocytes phenotyping looking for B cell lymphopenia and the frequent T cell associated abnormalities.