Rationale: General intensive care units provide care across a wide range of diagnoses while specialty ICUs provide diagnosis-specific care. Risk-adjusted outcome differences across such units are unknown. Objective: To determine the association between specialty ICU care and the outcome of critical illness. Methods: We conducted a retrospective cohort study design analyzing patients admitted to 124 ICUs participating in the Acute Physiology and Chronic Health Evaluation (APACHE) IV from January 2002 to December 2005. We examined 84,182 patients admitted to specialty and general ICUs with an admitting diagnosis or procedure of acute coronary syndrome, ischemic stroke, intracranial haemorrhage, pneumonia, abdominal surgery, or coronary-artery bypass graft surgery. ICU type was determined by a local data coordinator at each site. Patients were classified by admission to: (1) a general ICU, (2) a diagnosis-appropriate ("ideal") specialty ICU, or (3) a diagnosis-inappropriate ("non-ideal") specialty ICU. The primary outcomes were in-hospital mortality and ICU length of stay. Measurements and main results: After adjusting for important confounders, there were no significant differences in risk-adjusted mortality between general versus ideal specialty ICUs for all conditions other than pneumonia. Risk-adjusted mortality was significantly greater for patients admitted to non-ideal specialty ICUs. There was no consistent effect of specialization on length of stay, either for all patients or for ICU survivors. Conclusions: Ideal specialty ICU care appears to offer no survival benefit over general ICU care for select common diagnoses. Non-ideal specialty ICU care (i.e. "boarding") is associated with increased risk-adjusted mortality.