INTRODUCTION: Mucocele-like tumor was originally described in 1986 as a benign breast proliferation consisting of multiple dilated cysts lined by cytologically bland, flat to cuboidal cells. Subsequent reports described the coexistence of, including the morphologic inter-transitions between, mucocele-like tumor and a variety of other breast proliferations, including intraductal carcinoma, invasive carcinoma, atypical ductal hyperplasia, and hyperplasia of the usual type. The spectrum of breast alterations characterized by variably enlarged terminal-ductal lobular units lined by variably hyperplastic and variably atypical columnar cells has been the subject of significant discussion in the recent literature. In one scheme, these lesions may be classified into four groups, that is, columnar cell change with and without atypia and columnar cell hyperplasia with and without atypia. Morphologic and molecular observations suggest an association, perhaps in a nonobligate precursor role, between some columnar cell lesions and a variety of other neoplastic lesions. CASE PRESENTATION: We describe the case of a 43-year-old woman whose breast tumor contained areas diagnostic of mucocele-like tumor and columnar cell hyperplasia, with morphologic transitions in between. CONCLUSION: Our case represents the second broadly similar case that has been reported, and suggests a potential relationship between these two enigmatic lesions.