Neutrophil antibodies were demonstrated in 119 of 121 infants and young children with chronic neutropenia, establishing the diagnosis of autoimmune neutropenia of infancy. The median age at diagnosis was 8 months (range 3 to 30 months), and the female/male ratio was 6:4. Autoimmune neutropenia of infancy was manifested by recurrent fever and infection. All patients had selective neutropenia (absolute neutrophil count 0 to 500), and many had monocytosis. Fifteen of 16 patients tested failed to respond to epinephrine and hydrocortisone stimulation. Bone marrow had myeloid hyperplasia and reduced mature neutrophils. Recovery occurred in all 81 patients who passed the age of 5 years, except for one patient who is recovering at 6 1/2 years. The median age at recovery was 30 months; 95% recovered before 4 years. The estimated median duration of disease was 20 months. Neutrophil antibodies were detected early in the neutropenic phase by a combination of immunofluorescence and agglutination tests. Ten percent of these antibodies had specificity for NA1 or NA2. Ten of the 12 serum samples with a strong reaction in the flow cytometer reacted only with neutrophils. Two also reacted with an unidentified subpopulation (30%) of lymphocytes. Lymphocyte subsets were normal in 10 patients investigated, and abnormal levels of circulating immune complexes were detected in sera from 11 of 25 (44%) patients tested. Temporary remission was induced in all of eight patients who received intravenous IgG therapy. Autoimmune neutropenia of infancy is probably the most common cause of chronic neutropenia in infancy and early childhood, can be diagnosed by immunologic techniques, and requires only conservative management; spontaneous cure appears to be the rule.