Tropical landscapes represent storehouses of medicinal drug plants. It is widely held that these medicinal resources—real and potential—are threatened by a host of destructive forces. This paper examines these threats, especially the process of culture change and ethnobotanical erosion, in a rural Brazilian community. Employing a quantitative analysis of a sample plant pharmacopoeia, we investigate the relationship between medicinal plant knowledge and age, gender, and socio-economic standing. The results indicate that female gender, increasing age, illiteracy, and decreasing formal education are all positively correlated with level of medicinal plant knowledge. The process of modernization, particularly increasing access to formal education, appears to be incompatible with the retention of traditional domains of medical knowledge. Increasingly perceived as an irrelevant province of past generations, knowledge of the healing powers of tropical forests and fields is rapidly declining in this community.