The objective of this study is to explore the relationship between variables that may influence pharmaceutical costs in asthma and to generate a predictive model for these costs in primary health care. The understanding of these relationships is important since costs of drugs may place unnecessary economic burden on patients and society. During 2003, prospective clinical data were collected from 105 patients in 24 primary health care centers located in Stockholm. The relationships between cost of drugs and quality of life, lung function, and asthma severity were analyzed in a regression model. Twenty-three percent of the observed variation in pharmaceutical costs could be explained by asthma severity, disease-specific quality of life, and clinical practice. There was a weak inverse correlation between pharmaceutical costs, generic quality of life, and lung function. Even when severity was accounted for, there were large variations in costs between different primary health care units.