Objective: Mannose binding lectin (MBL) is a key molecule in first line defense against variou's microorganisms. Polymorphism of the MBL gene greatly affects serum MBL concentration, and is known to be associated with occurrence of infectious diseases. We aimed to determine whether there is a relationship between occurrence or severity of chronic periodontitis (CP) and polymorphism of the MBL gene. Patients and methods: Ninety-eight CP patients and 63 healthy subjects with no periodontitis were typed for the codon 54 polymorphism of the MBL gene by PCR-restriction fragment length polymorphism method. Results: Genotype frequencies of the codon 54 polymorphism were similar between patients and control subjects. When patients were categorized to mild, moderate and severe periodontits groups, possession of the low serum MBL concentration allele was a risk factor for having severe CP, as assessed by logistic regression analysis. Conclusion: Patients with MBL genotypes that cause lower serum MBL concentration may be at risk of having severe periodontitis. MBL gene typing may become useful to predict the prognosis of CP. © 2009 The Japan Society for Clinical Immunology.