OBJECTIVE: To compare the lymphocyte T CD4+ (CD4) response to combinations of antiretroviral therapy (ART) in HIV-1, HIV-2 and dually positive patients in West Africa. DESIGN AND SETTING: Collaboration of 12 prospective cohorts of HIV-infected adults followed in Senegal (2), Gambia (1), Mali (2), Benin (1) and Côte d'Ivoire (6). Subjects: Nine thousand, four hundred and eighty-two patients infected by HIV-1 only, 270 by HIV-2 only and 321 dually positive, who initiated an ART. OUTCOME MEASURES: CD4 change over a 12-month period. RESULTS: Observed CD4 cell counts at treatment initiation were similar in the three groups [overall median 155, interquartile range (IQR) 68; 249 cells/microl). In HIV-1 patients, the most common ART regimen was two nucleoside reverse transcriptase inhibitors (NRTIs) and one non-nucleoside reverse transcriptase inhibitor (NNRTI; N = 7714) as well as for dually positive patients (N = 135). HIV-2 patients were most often treated with a protease inhibitor-based regimen (N = 193) but 45 of them were treated with an NNRTI-containing ART. In those treated with a NNRTI-containing regimen, the estimated mean CD4 change between 3 and 12 months was significantly lower in HIV-2 (-41 cells/microl per year) and dually positive patients (+12 cells/microl per year) compared to HIV-1 patients (+69 cells/microl per year, overall P value 0.01). The response in HIV-2 and dually positive patients treated by another regimen (triple NRTIs or protease inhibitor-containing ART) was not significantly different than the response obtained in HIV-1-only patients (all P values >0.30). CONCLUSION: An optimal CD4 response to ART in West Africa requires determining HIV type prior to initiation of antiretroviral drugs. NNRTIs are the mainstay of first-line ART in West Africa but are not adapted to the treatment of HIV-2 and dually positive patients.