Introduction Lymphatic drainage patterns from the breast have been described in the past. Drainage may change after treatment of a breast or axilla, and this may have implications for lymphatic mapping. The aim of this study was to determine the lymphatic drainage patterns in breast cancer patients with a previously treated ipsilateral breast. Methods Between January 1999 and November 2008, 115 sentinel node procedures were performed in breast cancer patients who had undergone treatment of the ipsilateral breast in the past. Lymphatic drainage patterns were analyzed based on preoperative lymphoscintigraphy and sentinel lymph node biopsy. Patients were divided into subgroups according to their previous treatment. Results Sentinel nodes were found in 84% of the patients: in 81 patients (70%) in the axilla, 43 patients (37%) had drainage to more than one site, and in 18 patients (16%) no drainage was detected. The percentage of drainage outside the axilla was higher than in a series of untreated breast cancer patients from our institution (51% versus 33%, P  = 0.01). The 16% nonidentification rate was also higher than the 3.1% in patients without previous treatment ( P  = 0.003). Four patients (3.5%) had lymphatic drainage to the contralateral axilla. Twelve patients (10%) had involved sentinel nodes; these were harvested from the contralateral axilla in two of them. No lymph node recurrences were observed during a median follow-up time of 39 months. Conclusion Lymphatic mapping yields a lymph node in 84% of breast cancer patients who have undergone previous treatment of the breast. Nonidentification and extra-axillary nodes are more frequently encountered than in patients without treatment of the breast in the past. The finding of involved nodes suggests that sentinel node biopsy improves staging. Long-term follow-up will determine the sensitivity of the procedure in this specific situation.