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Thrombocytopenia occasionally follows immunization of children, especially after administration of measles-containing vaccines. The purpose of this study was to describe the clinical features of postimmunization thrombocytopenia, with emphasis on the rate of complications and outcome.A prospective survey was conducted by 12 pediatric centers in Canada during 1992 to 2001. At each center a nurse monitor searched for inpatient cases. Cases were defined as having onset of clinical signs or laboratory measures of thrombocytopenia (platelet count, <50 × 109/l) within 30 days after immunization. Cases were described in a standardized manner, including follow-up data as available.Sixty-one cases were detected, an average of 6 per year or ∼1 case per 15 000 general hospital admissions. Median age of cases was 13 months. The mean platelet count at diagnosis was 8.6 × 109/l. Most cases (79%) followed measles-containing vaccines. Only 1 child had a serious (fatal) complication. Platelet counts returned to normal within 30 days of onset in 46 of 57 children (80.7%) with information available. Five children (8.2%) had persistent or intermittent thrombocytopenia for 3 months or more.Thrombocytopenia associated with routine immunization of children is rare and usually benign, resolving within 1 month in most children.