Herbarium data sources are often the only available information on plant species. In this paper, the potential use of herbarium data is highlighted in a case study of the family Myrtaceae in the Atlantic forest of the Brazilian State of Espirito Santo, focusing on areas of conservation priority and assessment of species diversity. Occurrence records were obtained from published texts, personal records, and information from herbarium collections available on the SpeciesLink website. Authoritatively, determined herbarium collections were used to clarify doubts related to species identifications. A total of 4,296 records were examined, corresponding to 292 accepted species names and 106 synonyms. The variables species richness and number of collections were highly correlated. Many areas have been poorly sampled, especially in the northern and northwestern regions of Espirito Santo. Areas with high endemism and diversity rates were those most collected, suggesting that a large information potential in richness and endemicity parameters remains unsampled in poorly known areas. Eighteen endemic species were reported, of which Campomanesia espiritosantensis, Myrcia follii, M. limae, and M. riodocensis are those most vulnerable to extinction. In order to improve Myrtaceae conservation status, effective government support in implementing conservation policy is required, as well as expansion and creation of new protected areas. We suggest that Priority Areas for Conservation should be used as suitable sites for implementation of some kind of further protection.