Background:The aim of the study was to identify the characteristics and clinical course of acquired neutropenia with childhood infection. Methods: 215 children in two centers with neutropenia were enrolled in this study.The clinical course and laboratory features were investigated retrospectively. Results:215 previously healthy children(116 male [54%]), whose mean age was 36.08 months.210patients(97.7%) had post infectious neutropenia,while 5patients(2.3%) had autoimmune neutropenia.An infection agent was identified in 130/215(60.5%)cases(virus infection:n=102(47.4%),mycoplasma infection:n=18(8.4%),bacterial infection: n=10(4.7%)).The distribution of viruses was as follows; RSV(n=29,28.4%),Influenza(n=20,19.6%),Parainfluenza(n=16,15.7%).The mean absolute neutrophil count(ANC) at the time of diagnosis and on minimum value was 732.30 and 624.56x10sup＞6/sup＞/L.Patients with autoimmune neutropenia usually had lower ANC than those of post-infectious neutropenia(p=0.0368, 0.009).In the patients with post-infectious neutropenia,severity of neutropenia and ANC were significantly associated with a) infectious agents,b) virus types. a)Infection agents;mild neutropenia was mostly in virus infection,and moderate neutropenia was mostly in bacterial infection(p= 0.0162,0.0206).b)Virus types;mild neutropenia was mostly in RSV infections,in contrast, severe neutropenia was mostly in parainfluenza virus infections(p=0.0129,0.0174).Number of co-infected viruses was associated with severity of neutropenia in the patients with post-infectious neutropenia.The greater the number of infected virus,the less severe neutropenia was shown(p=0.0190).The mean duration of the neutropenia was 18days.The recovery of neutropenia was significantly associated with virus types.RSV infection was best recovered from neutropenia (HR:1),and both influenza(HR:0.33,CI:0.16-0.70,p=0.0042) and parainfluenza virus (HR: 0.33, CI: 0.15-0.74, p=0.0072) infection had tendency to be recovered less likely in comparison with RSV infection. Conclusion:Neutropenia in previously healthy children was usually transient,often following virus infection,which mainly subsided within 2 months and mainly of mild to moderate severity.But,patients with parainfluenza virus infection appeared to have severe neutropenia predominantly.Majority of children recovered spontaneously without serious complications.