Objective. Tuberculosis and HIV coinfection poses unique clinical and psychosocial complexities that can impact nonadherence to highly active antiretroviral treatment (HAART). Methods. This was a prospective case series to identify risk factors for HAART nonadherence among 43 patients with HIV and tuberculosis (TB) in Lima, Peru. Nonadherence was defined by patient self-report. Results. The median initial CD4 and HIV viral load were 63 and 159,000, respectively. Patients had received a median of 6.1 months of ART. Univariable analysis found low social support, substance use, and depression to be associated with nonadherence. In multivariable analysis, low social support was associated with nonadherence. Conclusions. In the authors' urban cohort of HIV-TB coinfected individuals in Lima, Peru, substance use, depression, and lack of social support were key barriers to adherence. These findings suggest that adherence interventions may be unsuccessful unless they target the underlying psychosocial challenges faced by patients living with TB and AIDS. © 2008 Sage Publications.