Adherence is a decisive factor in achieving a successful response to antiretroviral therapy (ART) for HIV infection. No previous studies have been conducted regarding HIV treatment adherence in Guinea-Bissau. In this study we assessed barriers and facilitators to patient ART adherence. Semi-structured interviews were conducted with 20 adult, HIV infected individuals receiving ART at a HIV treatment centre in Bissau, Guinea-Bissau. The grounded theory method was used to gather and analyse data. Results indicated that HIV-related knowledge was a determining factor for optimal adherence. The facilitators were experienced treatment benefits and complementing social networks. The barriers were treatment-related costs and competing livelihood needs; poor clinic infrastructure; perceived stigma; and traditional practices. Our findings indicate that good ART adherence, especially in resource-limited settings, requires that patients achieve adequate HIV-related knowledge. More studies on HIV-related knowledge and adherence among HIV infected individuals are currently needed.