INTRODUCTION: Medical care provided by medical specialists is one of the scarcest resources in the public system. It is costly and difficult to access for the general population. Availability and accessibility of specialized care is related to economic, social and cultural aspects that vary among geographical areas. An aggravating factor for this situation is patients’ failure to appear on the date of their appointment, which is defined as the nonattendance of patients to medical specialist appointments without notice. OBJECTIVES: To measure and analyze the phenomenon of nonattendance of patients to medical appointments with specialists in the public healthcare system of Chile and its relationship with environmental and socioeconomic regional indicators. METHODS: Ecological design study, using medical care records in the public system and environmental and socioeconomic regional indicators potentially related to the absence of patients, between the years 2005-2010. Poisson regression models with random components were used for assessing associations. RESULTS: There is 16.5% of nonattendance of patients, with a range between regions from 8.8 to 20.2%. Nonattendance is higher in the specialties of dermatology, geriatrics and nutrition (20.0%), in children (3.1% more than in adults), in areas with highest indigenous population (RR=1.3), in areas with low diversity of specialties (RR=1.1) and in the months of February, July, November and December (RR>1.1). CONCLUSIONS: In Chile, socioeconomic factors and the management of healthcare resources have greater influence on the nonattendance of patients to medical specialists’ appointments than environmental factors; therefore, this phenomenon may be avoidable. Abstract available from the publisher.