Despite extensive patient education, few heart failure (HF) patients master self-care. Impaired cognitive function may explain why patient education is ineffective. A concurrent triangulation mixed methods design was used to explore how knowledge and cognitive function influence HF self-care. A total of 41 adults with HF participated in interviews about self-care and completed standardized instruments measuring knowledge, cognitive function, and self-care. Content analysis uncovered themes suggesting that lack of understanding, not lack of knowledge, contributes to poor self-care. Linear regression tested the relative influence of knowledge and cognitive function on self-care. Cognitive function was a stronger determinant of self-care than knowledge. Poorer cognitive function was related to better self-care and explained in part by mixed methodology and the qualitative narratives. (Contains 7 tables and 1 figure.)