Chronic neutropenia comprises several different diseases that vary in degree of severity and management. We analysed the levels of the neutrophil-derived protein pro-LL-37 in plasma of patients with chronic neutropenia to assess whether it could be used to differentiate different categories of chronic neutropenia. All patients with severe congenital neutropenia were pro-LL-37 deficient. This was in contrast to patients with autoimmune or idiopathic neutropenia, who exhibited normal pro-LL-37 levels while patients with cyclic neutropenia displayed an oscillation of pro-LL-37 in plasma. Plasma levels of pro-LL-37 may thus prove useful for differential diagnosis of chronic neutropenia.