Local knowledge is becoming increasingly important in primary health care projects. However, these projects often incorporate local knowledge in an uncritical manner. One area where this is apparent is in the lack of attention paid to the gendered nature of local knowledge. I use one example, women’s knowledge and use of medicinal plants in a low-income community in the Brazilian Amazon, to illustrate the links among authority, knowledge, and gender. In this article I argue that policy makers must pay attention to the relationships among authority, gender, and local knowledge and examine how the use of local knowledge in development strategies can affect existing (gendered) power relationships. Women’s roles as managers of household health (which includes medicinal plant use) are a source of authority for them. Because of that, the way in which local knowledge is incorporated into primary health care programs can have a significant impact on women’s authority.