Introduction Recent guidelines suggest that ultrasonography should be used as the primary imaging modality in women under 40 years of age with mammography being offered if further imaging is required. The aim of this study was to assess the adequacy of ultrasonography and the utility of mammography in this patient group by reviewing the role these imaging techniques had in the diagnosis of breast cancer in our unit. Methods All breast cancers diagnosed in patients 39 years or younger from June 2009 to June 2011 were reviewed. This was a retrospective review of presentation, clinical findings, imaging modality (ultrasonography, mammography, magnetic resonance imaging [MRI]) and histology. Mammography was the primary imaging modality until May 2011 in women between 35 and 39 years of age. Both invasive and intraductal carcinoma were included in the study but lobular carcinoma in situ was excluded. Results A total of 2,495 patients were referred to the symptomatic breast clinic in this age group during the study period. Thirty women were identified with either invasive cancer (n=27) or ductal carcinoma in situ (n=3). Twenty-eight patients underwent mammography, graded as uncertain, suspicious or malignant in the majority. Malignancy was missed in one patient. All 30 patients underwent ultrasonography, reported as uncertain, suspicious or malignant, an indication for diagnostic core biopsy. Ultrasonography alone did not miss any cancers but did fail to detect multifocal disease in one patient. Conclusions In this study group, ultrasonography was reliable as the primary imaging modality for women under 40, identifying all cancers in this cohort. Mammography and/or MRI remain essential adjuncts to accurately determine multifocality and/or the extent of disease.