Ethnopharmacological relevance: The concepts of health and illness, and their causes, are fundamental for un- derstanding medicinal plant choice and use by traditional people. The hot-cold system is widespread in Meso- american traditional medicine and guides many therapeutic decisions. Aim of the study: This study explores a hypothesis that climate influences the hot-cold classification of illnesses and medicinal plants, and the perception of hazard of illnesses. In addition, we examine the classification cat- egories within the system used in different regions of Mexico. Materials and methods: Studies from Mexico with quantitative and qualitative data on the hot-cold properties of medicinal plants and ailments were reviewed. The information was organized and then related to the climate type of the study areas. Results: In temperate climates, most diseases were considered cold, and hot medicinal plants were dominant. Conversely, in warm-tropical climates, hot diseases dominated, and the majority of medicinal plants were cold; however, this evidence was weaker. The perception of hazard was congruent with the number of illnesses for temperate climates. There were additional classification categories within the hot-cold system for diseases and medicinal plants, and they were expressed in different terms in Spanish, English, and indigenous languages. Although similar terms and categories were used in the classification of diseases and medicinal plants, they can differ conceptually and vary between places and cultures. Publications are sometimes unclear if the terms used are emic or etic. The basic principle of using plants with the opposite property of the disease does not always apply strictly. Conclusions: Climate appears to influence the hot-cold classification of diseases and medicinal plants in Mexico, and the system is not strictly dual. Improved knowledge of the hot-cold system is necessary to understand Mesoamerican medicinal plant use and culture.