To determine the prognostic significance of signet ring cells in infiltrating lobular carcinomas, the percentage of signet ring cells in 99 infiltrating lobular carcinomas was correlated with the patients' clinical outcomes (mean follow-up interval of 4.8 years). When the carcinomas were divided into those with 0%, 1-9%, and 10% or more signet ring cells, 57% (26/46) of patients with 10% or more signet ring cells had experienced recurrences or metastases compared with 40% (2/5) and 31% (15/48) with 0% and 1-9%, respectively. A similar analysis performed with breakpoints at 20% or 30% failed to yield any statistically significant associations. When patients were stratified by pathologic stage, patients with stage I disease and 10% or more signet ring cells were more likely to have recurrences or metastases than those patients with stage I tumors and fewer than 10% signet ring cells. There was no relationship between signet ring cells and disease progression in stages II, III, and IV. These results indicate that the presence of 10% or more signet ring cells represents a poor individual prognostic factor in stage I infiltrating lobular carcinomas.