BACKGROUND A small percentage of previously healthy children develop a sudden overwhelming infection (SOI) that rapidly progresses and results in shock and, occasionally, death. Some of these children may have an undetected Ig deficiency. OBJECTIVE The aim of this study was to evaluate the incidence of Ig deficiency in children with a SOI. METHODS A case series study was conducted in a university hospital and included 18 children who either died in the emergency room or required admission to the pediatric intensive care unit secondary to a SOI. Two age-matched control groups included children hospitalized to regular floor beds with an infectious process (infected control group) or a noninfectious process (noninfected control group). Serum left from the initial blood draw, before fluid resuscitation, was collected and stored at -70 degrees C. Total IgG, IgG subclasses, IgM, and IgA were assayed by rate nephelometry in a blinded fashion. RESULTS In the study group, one of six children under 1 year of age had low Ig levels in comparison with two of nine control patients. In those children over the age of 1 year, 8 of 12 patients (67%) had low Ig levels as compared with 2 of 19 controls (11%) by Fisher's exact test, P = .002. Of those patients with Ig deficiencies, three of eight had isolated IgG deficiency, two of eight had combined IgG and IgA deficiency, three of eight had combined IgG and IgM deficiency. CONCLUSIONS Children over the age of 1 year who present with a SOI have a significantly higher incidence of Ig deficiencies compared with age-matched controls.