BACKGROUND There are few studies examining breast cancer in women under the age of 40 years, particularly in western European populations. Such tumours are reported to be more aggressive, possibly due to a different pathophysiology compared to older patients. METHODS We performed a retrospective review of all women less than 40 years of age, diagnosed or treated with breast cancer, from June 2001 to June 2007 to assess pathophysiological factors that may influence clinical outcome and prognosis including patient demographics, clinical presentation, pre-operative investigations, surgical and pathological findings, treatment and outcome. RESULTS Fifty-eight women (mean age 34.9 years, range 27-39 years) were identified. One patient was excluded due to incomplete data; 98.2% (n=56) patients presented directly to our symptomatic clinic; 89.5% (n=51) patients had a palpable lump; 71.9% (n=41) patients had no family history. Mammography was less sensitive than ultrasound (64.3% vs. 82.4%) while fine needle aspiration cytology was 92.5% sensitive for malignancy. Twenty-nine (50.9%) patients underwent breast-conserving surgery (BCS) of which 7 proceeded subsequently to completion mastectomy due to involved margins. Twenty-six (45.6%) patients required total mastectomy primarily while 2 (3.5%) patients were treated palliatively due to metastatic disease. The mean tumour size (nearest resection margin) was 2.13cm (2.58mm) for BCS and 3.95cm (6.38mm) for mastectomy. From a total of 55 primary resections, 85.5% (n=47) of tumours were invasive ductal carcinoma; 57.4% (n=31) and 40.7% (n=22) were grade II and III tumours respectively. Lymphovascular invasion was identified in 50.9% (n=28) while 40.0% (n=22) were lymph node positive for metastatic disease. 76.8% (n=43), 39.3% (n=22) and 30.2% (n=16) were oestrogen, progesterone and human epidermal growth factor receptor-2 positive respectively. The mean Nottingham prognostic index was 4.37 (range 2.2-8.4). Neo-adjuvant and adjuvant chemotherapy was administered to 9.3% (n=5) and 80.0% (n=44) of surgically treated patients respectively while 76.4% (n=42) patients received adjuvant radiotherapy. 76.4% (n=42) of patients were treated with tamoxifen. Four patients received Herceptin therapy. Statistically significant univariate factors adversely associated with overall survival were time from referral to out-patient department attendance (p=0.038), administration of neo-adjuvant treatment (p=0.019), surgical intervention (p<0.001), progesterone receptor positivity (p=0.018) and tumour recurrence (p<0.001). 86.0% (n=49) patients were alive at mean follow-up of 52 months; 82.5% (n=47) remain disease free. CONCLUSIONS Our study reports a low familial trait rate combined with a high proportion of hormonally active tumours less than grade III which suggests that breast cancer in this series of young women from Northern Ireland may be less aggressive and more hormonally responsive than anticipated.