This is an evaluative study of the quality of vaccine storage and conservation in primary health care centers (PHC) in the mid-west region of the state of Minas Gerais. Dimensions, structures and processes were the criteria used to measure the level of quality in 261 vaccine storage and conservation units in over 55 municipalities in the area. Quality levels were defined by means of a scoring system with different weighted scores attributed to indicators for each dimension being rated. Categories for quality levels were then defined as: "adequate," "inadequate" and "critical." Pearson's chi-square test was used to verify the correlation between quality level and population size and adherence to the Unified Health System. It was observed that vaccine storage and conservation quality levels were inadequate in 59.3% and critical in 26.9% of these facilities. Small municipalities that are registered for Full Primary Health Care Management featured the worst vaccine storage facilities. Vaccine conservation 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 further studies in the field.