Suppression of viral replication is followed by increases in CD4+ lymphocytes, and this has been shown to result in decreased susceptibility to opportunists after initiation of highly active antiretroviral therapy (HAART). However, clinical aggravations after the initiation of HAART have been thought to be due to the restored ability to mount an inflammatory response, or the immune reconstitution inflammatory syndrome (IRIS). The degree of IRIS observed in human immunodeficiency virus (HIV)-infected patients following initiation of HAART is variable. This prospective study was aimed at determining the proportion of IRIS and the pattern of opportunistic infections among 186 HIV/AIDS patients receiving HAART between December 2006 and July 2007 at Zewditu Memorial Hospital, Addis Ababa, Ethiopia. The proportion of IRIS was 17.2% (32/186). The mean number of days of IRIS occurrence for each disease ranged from 26 to 122 days with a mean of 80. Opportunistic diseases associated with IRIS were tuberculosis (68.8%, 22/32), herpes zoster rash (12.5%, 4/32), cryptococcosis (9.4%, 3/32), toxoplasmosis (6.3%, 2/32) and bacterial pneumonia (3.1%, 1/32). Compared to baseline readings there were significant increases in CD4 count, aspartate aminotransferase and alanine aminotransferase levels while hemoglobin values decreased during the development of IRIS. In summary, the proportion of IRIS and the pattern of opportunistic infections in HAART-treated patients in Ethiopia mirrored those reported in other countries. Further prospective surveys on epidemiological, immunological, microbial and clinical studies are imperative to assess the proportion and pattern of IRIS and effect of HAART in Ethiopia.