Introduction: Leukocyte adhesion deficiency (LAD), type I is a rare autosomal recessive disorder caused by a ITGB2 gene mutation on chromosome 21 which encodes the β2 integrin molecule (CD18). Over 300 cases have been reported worldwide. It is characterized by delayed umbilical cord separation, marked leukocytosis and recurrent infections. We report an adolescent with LAD1, diagnosed by flow cytometry and diagnostic exome sequencing. Case report: A 16-year-old girl was admitted complaining of fever, abdominal pain and diarrhea. She was tall and slender with weight 3 percentiles. She had a history of delayed cord detachment (3 weeks) and has suffered from frequent infections, including otitis, pneumonia, skin infections in younger age, and persistent gingivitis. She had significant leukocytosis, and was suspected to have LAD at the other hospital. On admission, evidence of severe colitis with reactive lymph node hyperplasia were seen in abdominal CT scan. Blood counts were: WBC, 95,300/μL with 90% neutrophils; Hgb, 7.7 g/dL; MCV, 71.7 fL; platelets, 370k/μL. Serum CRP and procalcitonin were significantly elevated. Cultures were negative, but stool was positive for Clostridium by PCR. Serum immunoglobulines, lymphocyte subsets and tests for chronic granulomatous disease were all normal. Her condition improved with antibiotics and supportive care. Flow cytometric assessment of CD18 and CD11b were 0.2% and 3.5% of normal control, respectively, consistent with severe phenotype of LAD type 1. Two compound heterozygous mutations in the ITGB2 gene were detected by diagnostic exome sequencing: a previously reported missense mutation in exon 3 (c.533C＞T, p.Pro178Leu) and a novel frameshift mutation in exon 6 (c.82_95del, p.Phe28Leufs*26). The former likely pathogenic variant was identified in her mother, and the latter was from father. Both were confirmed by Sanger sequencing. As she has survived her childhood, she is not undergoing immediate stem cell transplantation although her unaffected sister is HLA-identical to her. Currently, she is well on prophylactic antibiotics. Conclusion: LAD type 1 should be suspected in infants with delayed separation of umbilical cord, and frequent infections. Prompt initiation of antibiotics and early stem cell transplantation for patients with severe phenotype should be pursued.