The objective of this study is to clarify use of the term mastitis in empirical studies of lactational mastitis. Eighteen articles published between 1998 and 2008 were critically analyzed, and a concept clarification of the term mastitis was carried out. Four articles suggested that mastitis was an infection, 4 made no mention of the etiology of symptoms, and 10 articles described mastitis as an inflammation or an infection. Eight studies made no use of distinctions between milk stasis, noninfectious mastitis, and infectious mastitis. Bacterial cultivation was used in 5 articles, and 1 article measured leukocytes and C-reactive protein. Several different causes were cited. Antibiotics were suggested as appropriate treatment in 4 articles. Use of the concept of mastitis in empirical studies remains inconsistent and may do so until the etiology of symptoms is understood. Understanding is also needed as to why some women who present with symptoms suggesting septic reaction recover without antibiotic therapy.