This study sought to assess the quality of vaccine storage in Primary Healthcare Centers -PHC in the mid-west area of the state of Minas Gerais. To this end a twostage assessment study was carried out. In the first stage, dimensions, structures and processes were the criteria used to measure quality grades in 261 vaccine rooms distributed over 55 municipalities in the area and to analyze its relation to population size and adherence to the Unified Health System. Quality grades were defined by means of a scoring system, with different weighted scores attributed to indicators for each dimension being rated. Categories for quality grades were then defined as: "adequate", "inadequate" and "critical". Pearson's chi-square test was used to verify the correlation between quality grade and population size and adherence to the UHS. In the second stage of the investigation, vaccine rooms meeting 100% of the selected criteria in the structure dimension were selected. Thus, 12 vaccine rooms were selected for the conduction of semi-structured interviews with nurses, nursing assistants and technical immunization coordinators with the aim of shedding light on the factors which render vaccine storage in PHCs difficult. On analysis of the results, it was observed that vaccine storage quality grades are inadequate in 59.3% of these centers, and critical in 26.9%. Small-sized municipalities qualified for Full Basic Healthcare Management aggregated the worst vaccine rooms. With respect to the structure dimension, one of the main findings was absence of professionals with up-to-date knowledge on immunization in the vaccine rooms, which jeopardizes the delivery of quality healthcare assistance in vaccine rooms. With reference to the vaccine storage process dimension, the refrigerator organization component achieved the worst quality evaluation, chiefly concerning the organization of the vaccines in a manner in which they are maintained at stabilized temperatures. The study points to an absence of knowledge on the parts of the nurses and nursing assistants as to the effects of low temperatures on vaccines. Additionally, breached cold chains did not score well; most vaccine room staff fail to follow guidelines when the immunobiologicals are subjected to temperature breaks. Limitations were also encountered in relation to vaccine-room task supervision. Excessive demands on the nurse's time and inadequate organization of work processes ultimately hamper the nurse's ability to exercise his/her primary role of healthcare management, represented here by vaccine room supervision. Vaccine storage supervision, which spans a series of activities present in nursing praxis, indicates the need to train human resources, monitor and assess work processes and conduct new studies in the field.