Lyons et al. have shown that human postpartum breast cancers, defined as cancers that arise up to five years after childbirth, have increased peritumour lymphatic vessel density. This characteristic may underlie increased rates of metastasis in women with postpartum breast cancers compared with breast cancers in other cohorts. The authors showed that in rodent models of this cancer, increased peritumour vessel density is due to overexpression of cyclooxygenase 2 (COX2) by tumour cells and treatment of these tumours with COX2 inhibitors caused a significant reduction in tumour cell invasion and metastasis. These results suggest that COX2 inhibitors are promising therapeutics for postpartum breast cancers.