The influence of patient age on risk of recurrence in the breast was retrospectively studied in 496 stage I-II invasive ductal carcinomas treated by macroscopically complete primary tumor excision followed by radiotherapy. With a median follow-up of 71 months, local recurrence occurred in 13 of 62 (21%) patients younger than 40 years, compared with 48 of 434 (11%) older patients (P less than .025). Cox multivariate analysis of 18 parameters identified four that significantly determined risk: major lymphocytic stromal reaction (MCR), unsatisfactory resection margins, increasing histologic grade, and extensive intraductal cancer (DCIS) within the primary tumor. Compared with older patients, those younger than 40 years had tumors that more often exhibited MCR (36% v 20%, P less than .01), histologic grade 3 (42% v 28%, P less than .025), and very extensive DCIS (21% v 6%, P less than .001). The status of resection margins did not differ significantly between younger and older patients. Restriction of Cox analysis to patients younger than 40 indicated that risk was adequately described by MCR and percentage of DCIS, without consideration of grade or margins. For patients younger than 40, local failure occurred in four of five (80%) tumors with both MCR and more than 50% DCIS, in eight of 25 (32%) with either, and one of 32 (3.1%) with neither of these morphologic features. This study suggests that the higher local failure risk observed in patients younger than 40 years reflects the greater prevalence of certain morphologic characteristics in breast cancers in younger patients. Age itself does not appear to be an independent determinate of risk.