Heterozygous mutations of the gene encoding neutrophil elastase (ELA2) have been associated with cyclic neutropenia (CN) and severe congenital neutropenia (SCN). To date, 30 different mutations have been reported, but no correlation has been found with the degree of neutropenia. To address this issue, we analyzed the clinical, hematologic, and molecular characteristics of 81 unrelated patients with SCN (n = 54) or CN (n = 27). We identified mutations in 31 patients, two thirds of whom had sporadic forms. Familial cases were consistent with dominant inheritance. Seventeen novel mutations were identified, showing that the mutational spectrum encompasses not only the region encoding the mature enzyme but also the prodomains and promoter region. Genotype-phenotype analysis strongly suggested that ELA2 mutations correlate with more severe expression of neutropenia, specifically in patients diagnosed with SCN. This study underlines the importance of ELA2 molecular screening to identify patients who may be at particular risk of severe bacterial infections and/or acute myeloid leukemia/myelodysplasia. By phenotypic analysis of affected relatives and carriers of the same ELA2 mutations, we showed that the expression of neutropenia in CN and SCN may be either homogeneous or variable according to the type of mutations, suggesting different pathogenetic mechanisms.