Background: Hyper-IgM syndrome (HIGM) is a rare primary immunodeficiency used to describe a heterogeneous group of disorders characterized by recurrent bacterial infections, normal or elevated serum IgM levels and low or absent serum IgG, IgA and IgE. Aim: To make definitive diagnosis, detect mutations in carriers and perform genetic counseling in patients with HIGM. Patients and methods: We studied the expression of CD40L, CD40 and made a mutation analysis of the CD40L gene in 3 males of 2 unrelated Chilean families diagnosed as a possible syndrome of hyper-IgM and 3 relatives. Results: We identified a deletion frameshift in the exon 2 (delA225) of the extracellular domain of CD40L gene in one patient and verified the carrier status of his mother and sister. The other patients showed a low expression of CD40L in activated T cells (65.3% and 65.5%) and a normal expression of CD40. No alterations were found in the single strand conformation polymorphism analysis of the CD40L. Conclusions: These result allowed us to make a definitive diagnosis of HIGM1 of a patient, detect female carriers and suggest a HIGM of recessive inheritance with normal CD40 expression in the patients of the second family (Rev Méd Chile 2004; 132: 1179-88)