National immunodeficiency registers in several countries have reported prevalence but not incidence rates for severe combined immunodeficiency (SCID). The objective of this study was to document the incidence and type of SCID in Australia, the age and clinical features at presentation and short-term management. The Australian Paediatric Surveillance Unit conducts active, monthly, national surveillance of rare disorders with reporting by child-health specialists. Between May 1995 and December 2001, clinicians provided clinical and laboratory data on children newly diagnosed with SCID. Thirty-three incident cases of SCID were identified [incidence 1.8/10(5) live births per annum; 95% confidence interval (CI) 1.2-2.5]. Twenty-six children had classical SCID (1.45/10(5) live births; 95% CI, 0.9-2.0) and 20 (77%) of these were boys. Classical SCID was X-linked in 13 children, autosomal recessive (AR), not further classified in four, and attributed to adenosine deaminase deficiency (ADA) in four, interleukin-7 receptor alpha chain deficiency in one, Ommen syndrome in two, and Di George syndrome in two. Twenty-one (81%) received bone marrow/stem cell transplantation, three of whom died between 1 and 4 months after transplantation (two ADA deficient, one AR). Seven children had atypical SCID, five of whom died within 1-4 yr of diagnosis. Most children with SCID presented with failure to thrive and recurrent infections and there was no significant delay between presentation and diagnosis. The reported national incidence of classical SCID in Australia (1.45/10(5) or approximately 4 cases per year or 1/69,000 live births) is consistent with the rate (1.4/10(5)) previously reported in Victoria. Diagnosis is rarely delayed, and transplantation is associated with good short-term survival.