Mannose-binding lectin (MBL) plays an important role in the innate immune response. Three alleles in the MBL gene, and one allele of the promoter, independently cause low serum MBL levels as compared with the wild-type. This study investigated the relationship between MBL genotype and the occurrence of nosocomial infection among neonates in a neonatal intensive care unit (NICU). Prospectively gathered information concerning nosocomial infection was available for 742 neonates from a recently performed surveillance study in an NICU. DNA was isolated from Guthriecards for a subgroup of 204 neonates who stayed in the NICU for > or =4 days. After a pre-PCR for the MBL gene in blood spots on Guthriecards, mutations were analysed by real-time PCR to detect six mutations in the MBL gene. An MBL genotype could be determined for 186 neonates. As compared to term neonates, genotypes encoding MBL-deficient haplotypes were significantly more prevalent among pre-term neonates. Forty-one of these neonates developed sepsis, with blood cultures yielding coagulase-negative staphylococci in 25 cases. Pneumonia occurred in 30 cases, with various causative organisms. No relationship was found between MBL genotype and the risk of nosocomial sepsis or pneumonia, even after correction for birth-weight, perhaps because of an insufficient correlation between genotype and the concentration of functional MBL. In addition, most bloodstream infections in the NICU were caused by coagulase-negative staphylococci, to which MBL binds poorly.