This study was nested within the HIV Outpatient Study cohort and investigated the incidence and risk factors for immune reconstitution inflammatory syndrome (IRIS) and the impact of IRIS on mortality. IRIS was defined as a new type B or C AIDS-defining condition or one of a range of mucocutaneous or autoimmune conditions diagnosed within 180 days of starting a new combination antiretroviral therapy regimen, provided there was a documented HIV viral load or CD4 response. IRIS occurred in 10.6% of the 2610 patients. Risk factors independently associated with IRIS included high HIV viral load and low CD4 count. IRIS related to type B or C AIDS-defining conditions was associated with subsequent mortality. Deaths among IRIS cases occurred over 3 years (median) after the IRIS event, making it unlikely that these deaths were directly attributable to IRIS. The IRIS case definition used has important differences when compared with previously published IRIS case definitions thereby affecting comparability of these findings, but this was a pragmatic definition for a large multicenter cohort study utilizing a central database.