Consultation with traditional healers (THs) is common among people living with HIV in sub-Saharan Africa. We conducted a prospective longitudinal study to determine the association between consultation with THs and HIV outcomes following 12 months of antiretroviral therapy (ART). HIV-infected individuals presenting for care and initiation of ART in Dakar and Ziguinchor, Senegal were eligible for enrollment. Data were collected using interviews, clinical evaluations, laboratory analyses, and chart reviews at enrollment, 6 months after ART initiation, and 12 months after ART initiation. Among the 186 participants, 35.5% consulted a TH. The most common reason for consulting a TH was "mystical" concerns (18%). Those who consulted a TH before ART initiation were more likely to present with a CD4 count < 200 cells/mm3 (44% versus 28%; P = 0.04) and WHO stage 3 or 4 disease (64% versus 46%; P = 0.03), and they were less likely to disclose their HIV status (44% versus 65%; P = 0.04). Those who consulted a TH more than 6 months after ART initiation were more likely to report poor adherence to ART (57% versus 4%; P < 0.01). The strongest predictor of virologic failure was consulting a TH more than 6 months after ART initiation (odd ratio [OR], 7.43; 95% CI, 1.22-45.24). The strongest predictors of mortality were consulting a TH before ART initiation (OR, 3.53; 95% CI, 1.25-9.94) and baseline CD4 count < 200 cells/mm3 (OR, 3.15; 95% CI, 1.12-8.89). Our findings reveal multiple opportunities to strengthen the HIV care cascade through partnerships between THs and biomedical providers. Future studies to evaluate the impact of these strategies on HIV outcomes are warranted.