The Classic hyper-IgM syndrome (HIGM) is the X-linked disease caused by mutation in the CD40 ligand gene on chromosome Xq26.3. Patients with HIGM present opportunistic or recurrent respiratory or gastrointestinal infections due to dysgammaglobulinemia during early childhood. A 5 month-old boy was admitted because of suddenly developed peripheral cyanosis for 1 day. He was born with 39+6 weeks of gestational age and 4.49kg (90th-97th percentile). His mother showed positive rheumatoid factor without any symptoms. On admission, he was showed peripheral cyanosis with tachypnea and coarse breathing sound without rale. Initial laboratory data showed hemoglobin 16.5 g/dL, white blood cell counts 22,250/μL, and 536,000/μL of platelet. Blood chemistry was normal with low C-reactive protein. Radiologic findings showed diffuse opacity of alveoli and interstitial inflammation. We began ventilator care with empirical antibiotics. Pneumocytis jiroveci was isolated by bronchoalveolar lavage. Other respiratory virus or mycoplasma study was negative. On immunologic studies, serum IgG and IgA were low, but IgM was normal in his age. And CD3 counts was 2307/μL (37.3%), CD19, 3816/μL (61.7%), CD4, 528/μL (22.9%), CD8, 302/μL, CD56, 37/μL (0.6%). The ultrasonographic findings for hepatobilliary system was not any abnormalities. We identified new mutation of CD40 ligand to premature termination (c.89[6]; Leu32Serfs*17) by direct sequencing. His mother was shown as carrier, but his maternal grandmother and mother\'s sister don\'t have the mutation. He received regular intravenous immunoglobulin and prophylactic antibiotics and antifungal therapy. He was alive with mixed chimerims after unrelated hematopoietic stem cell transplantation. This case was X-linked hyper IgM syndrome from new mutation of CD40 ligand to premature termination on exon 1 (c.89[6]; Leu32Serfs*17).