Mucinous mammary carcinoma (MC) is a tumor type with relatively favorable prognosis. Unlike the circumstances surrounding conventional invasive duct carcinoma, data are limited regarding precursor lesions for MC. This study characterizes patterns of mucinous ductal carcinoma in situ (DCIS) as a precursor lesion for MC. All slides from 130 cases of MC encountered between 2000 and 2011 at Henry Ford Hospital, Detroit, MI were reviewed to subclassify MC, identify DCIS, and explore transition patterns from DCIS to MC. Calponin, p63, chromogranin, synaptophysin, CD56, and MIB-1 immunostaining analyses were performed in 65 cases. Among 106 cases of pure (71 type A, 35 type B) and 24 cases of mixed MC, DCIS appeared in 88 (68%) specimens, with all but 4 showing luminal mucin accumulation. Dominant patterns of mucinous DCIS were cribriform/solid (66), cribriform and papillary (7), papillary (5), micropapillary (3), and flat (3). Fifty-seven (68%) cases of mucinous DCIS demonstrated transitions from DCIS to MC. Luminal mucinous distention, focal flattening and attenuation of the epithelium, and disruption of the duct wall resulting in a mucocele-like extravasation of malignant epithelia with escaping mucin was a transition pattern seen with all architectures of DCIS and in all types of MC. This was the only pattern of transition to type A MC. The epithelial outpouching, formation of a cleft with accumulation of mucin around the epithelium, and transition into mucin pools with floating tumor cell clusters was the second transition pattern that went from cribriform/solid DCIS to type B and mixed MC. DCIS preceding aggressive phenotypes of MC (type B and mixed) more often had a cribriform/solid architecture, higher nuclear grade, and higher Ki-67-labeling index (all P<0.05). In summary, mucinous DCIS is a precursor to MC with distinctive features that link patterns of DCIS with aggressive MC phenotypes. The 2 observed transitions between mucinous DCIS and MC suggest that pathogenesis of different types of MC is different correlating with less or more aggressive behavior of the latter.