In patients with HIV-1 infection who are starting combination antiretroviral therapy (ART), the incidence of immune reconstitution inflammatory syndrome (IRIS) is not well defined. We did a meta-analysis to establish the incidence and lethality of the syndrome in patients with a range of previously diagnosed opportunistic infections, and examined the relation between occurrence and the degree of immunodeficiency. Systematic review identified 54 cohort studies of 13 103 patients starting ART, of whom 1699 developed IRIS. We calculated pooled cumulative incidences with 95% credibility intervals (CrI) by Bayesian methods and did a random-effects metaregression to analyse the relation between CD4 cell count and incidence of IRIS. In patients with previously diagnosed AIDS-defining illnesses, IRIS developed in 37·7% (95% CrI 26·6–49·4) of those with cytomegalovirus retinitis, 19·5% (6·7–44·8) of those with cryptococcal meningitis, 15·7% (9·7–24·5) of those with tuberculosis, 16·7% (2·3–50·7) of those with progressive multifocal leukoencephalopathy, and 6·4% (1·2–24·7) of those with Kaposi's sarcoma, and 12·2% (6·8–19·6) of those with herpes zoster. 16·1% (11·1–22·9) of unselected patients starting ART developed any type of IRIS. 4·5% (2·1–8·6) of patients with any type of IRIS died, 3·2% (0·7–9·2) of those with tuberculosis-associated IRIS died, and 20·8% (5·0–52·7) of those with cryptococcal meningitis died. Metaregression analyses showed that the risk of IRIS is associated with CD4 cell count at the start of ART, with a high risk in patients with fewer than 50 cells per μL. Occurrence of IRIS might therefore be reduced by initiation of ART before immunodeficiency becomes advanced.