We studied the clinical, histologic, histochemical, and immunocytochemical characteristics of 61 mucinous tumors (38 pure, 23 mixed) retrieved from a consecutive series of 1,689 infiltrating carcinomas of the female breast. The only statistically significant predictors of favorable survival were histologic (pure) type coupled with the absence of axillary lymph node metastases. Other factors, including classification into A and B types according to Capella et al., and neuroendocrine status, as assessed by the presence of argyrophilia, granins, neuron-specific enolase (NSE), and synaptophysin (SYN),-all had no influence on survival. Argyrophilic cells were found in 16 pure mucinous tumors (42%) and in the mucinous component of four mixed tumors (17%). Granin (chromogranin A or B), NSE, and SYN immunoreactivities were demonstrated in all the argyrophilic tumors. We also found NSE- and SYN-immunoreactive cells in 31 of 41 and 16 of 41 nonargyrophilic (granin-unreactive) mucinous tumors, which supports the view that mucinous carcinomas of the breast as a whole are neuroendocrine-programmed tumors.