BACKGROUND Breast cancer is one of the most common cancers worldwide, and anxiety is a psychological morbidity that is inevitable. Many researchers have investigated its prevalence and detrimental effects, yet little is known when comparing the different breast cancer treatments. A systematic review of all available literature was indicated to encourage better understanding of anxiety in patients undergoing treatment for breast cancer. OBJECTIVES This review aimed to determine the best available evidence on the level of anxiety among women with breast cancer undergoing/had undergone cancer treatment(s), and factor(s) contributing to anxiety in these treatment modalities. INCLUSION CRITERIA Types of participants Women with breast cancer of stage 0 to stage IIIA breast cancer, over and equal to 21 and below the age of 65 years of age.Types of intervention Women who were undergoing/had undergone cancer treatment restricted to chemotherapy, radiotherapy, surgery or combined treatments, and were without other medical co morbidities.Types of outcomes A variety of outcome measures were used to assess anxiety in the included papers. Hospital Anxiety and Depression Scale and State-Trait Anxiety Inventory were mostly used.Types of studies This review considered quantitative papers (randomized control trials, descriptive studies and systematic review) that fulfilled both requirements: SEARCH STRATEGY: The search sought to gather data from published and unpublished studies conducted between 1990 and 2010. An initial search on CINAHL and Medline was done to identify relevant search terms. A search strategy was then developed, using MeSH headings and keywords. Following databases were searched: CINAHL; PubMed; ScienceDirect; PsycINFO; Cochrane Database of Systematic Review; Scopus; Wiley Interscience and PsycARTICLES. DATA COLLECTION/EXTRACTION Two reviewers independently assessed the eligibility of the papers for inclusion. Eighteen papers were selected based on relevance, and underwent assessment for methodological quality using MAStARI. Eleven research papers that met the level of methodological standard were included into the review. Both reviewers came to the same consensus on the included and excluded papers. DATA ANALYSIS Due to the methodological heterogeneity of the included papers, a meta-analysis was not possible. The studies were hence presented in narrative summary. RESULTS Anxiety seems to be ubiquitous, presenting itself in all treatment types for breast cancer. Anxiety level in breast cancer women who underwent chemotherapy was highest before the first chemotherapy infusion, mediated by age and trait anxiety. Radiotherapy regimes did not affect anxiety level in radiotherapy-treated patients, and most research concluded that anxiety level was higher among women who underwent mastectomy than breast conservation therapy. When compared, patients who underwent chemotherapy were more anxious. CONCLUSIONS The prevalence and intensity of anxiety has been shown to be pronounced among the three treatments. Chemotherapy, as compared to other treatments, has shown to be associated with a higher anxiety level. IMPLICATIONS FOR RESEARCH With the prevalence, intensity and correlated factors of anxiety identified through this review, future research may investigate the interventions that could help alleviate anxiety among these patients. IMPLICATIONS FOR PRACTICE Anxiety is prevalent in women with breast cancer undergoing treatment, especially those undergoing chemotherapy. Healthcare professionals should pay greater attention to identify cues of anxiety in patients and prevent/alleviate it.