Our purpose was to investigate a possible relationship between occurrence of type I diabetes and polymorphism of the mannose binding lectin gene. Polymorphism of codon 54 of the mannose binding lectin (MBL) gene, whose presence of the minority allele leads to significant reduction of serum MBL concentration, was investigated in 128 Japanese patients with type I diabetes and 78 healthy volunteers by restriction fragment length polymorphism method. Frequencies of the minority allele were compared between the patient group and the control group. Frequency of the minority allele was 24.2% in the patient group and 19.9% in the control group. The probability of being heterozygous or homozygous for the minority allele was 41.4% in the patient group and 33.3% in the control group. Patients with DRB1*0405-DQB1*0401 and/or DRB1*0901-DQB1*0303 haplotypes, the two major type I diabetes-prone human leukocyte antigen haplotypes, showed a slightly higher probability of being heterozygous or homozygous for allele B of the MBL gene. Possession of the minority allele of the MBL gene may be a minor risk factor for having type I diabetes.