Background: Diarrhoea is a common clinical presentation of gastrointestinal infections. Inpatients with HIV/AIDS, the etiology of the diarrhoea could be either parasitic, bacterial, fungal,enteric virus or HIV itself. Amongst the causes of diarrhoea, parasitic infections have beenreported to be considerable causes of morbidity and mortality in HIV/AIDS patients worldwide.rnObjective: The aim of this study was to determine the prevalence of diarrhoea associatedparasites in HIV sero-positiveand sero-negative adult patients presented with diarrhoea.rnMethods: Comparative cross-sectional study was carried out from June 2009 to January 2010among 143 HIV sero-positive and 129 sero-negative adult patients presenting with diarrhoea inGondar, Northwest Ethiopia. Stool samples collected from the study subjects were tested bydirect saline and iodine wet mounts for the detection of trophozoites and/or cysts of protozoa andova or larvae of helminthic parasites. For the detection ofCryptosporidiumspecies andIsosporabelli,modified Zeihel Neelsen Staining method was used. For identification ofMicrosporidiumspecies, modified trichrome staining was used. CD4+ cell counts for the HIV sero-positives wereestimated using FACS count and the HIV sero-status for comparison groups was done followingthe national HIV/AIDS screening test algorithm.rnResults: Sixty one (89.7%) of the HIV sero-positive and 7(10.3%) of the sero-negative patientspresented with chronic diarrhoea. The parasite yield was 35.8% and50% in patients with acuteand chronic diarrhoea respectively. That is, 50% of patients with chronic diarrhoea had at leastone parasite species identified as compared to patients with acute diarrhoea (35.8%). The overallprevalence of intestinal parasites in the HIV sero-positive and sero-negative patients were 33.6%and 45.7%, respectively. In the HIV sero-positive patients,Strongyloides stercoralisandGiardialambliawere found 14(9.8%) patients each followed byCryptosporidiumspecies in 12(8.4%). Inthe HIV sero-negatives, Entamoeba histolytica/dispar complex were predominant found in29(22.5%) followed byGiardia lamblia, 17(13.2%) andSchistosoma mansoni,9(6.9%). Theopportunistic parasitesCryptosporidiumspecies 12(8.4%) andMicrosporidiumspecies 2(1.4%)were found in the HIV sero-positives exclusively. NoIsospora belliwas detected in both groups.The prevalence of Cryptosporidium species tended to be higher in HIV sero-positive patientspresenting with chronic diarrhoea and in those with lower CD4rn+rncell counts. Of the HIV sero-positive patients, 74(51.7%) were on ART. The parasite prevalence was 18 (37.5%) and 30(62.5%) in the ART and pre-ART patients, respectively.rnConclusion: The study highlights the magnitude of parasitic etiologies of diarrhoea in HIV sero-positive and sero-negative adult patients. The helminthic parasiteStrongyloides stercoraliswasmore prevalent in the HIV sero-positive patients as compared with the sero-negatives (p=0.01).HIV sero-positive diarrhoeal patients with lower CD4rn+rncounts (