OBJECTIVES: Pharmaceutical care aims at preventing and reducing drug-related problems (DRP). The aim of this project was to investigate DRP in breast cancer patients as well as to compare two DRP-categorisation systems. METHODS: In December 2007 a cross-sectional prospective survey was conducted in a German gynaecological-oncology outpatient ward. DRP were identified using patient interviews and chart reviews. The identified DRP were categorised using the PI-Doc® and PCNE system. Subpopulations were evaluated using variance analyses, interrater agreement using Cohen’s Kappa (ê). RESULTS: The analysed 36 patients developed on average 7.36 DRP per patient. Most frequent DRP were adverse drug reactions (ADR) (70.94 %), interactions (11.70 %) and non-compliance (4.53 %). Common DRP in the category ADR were gastrointestinal (27.13 %), dermatological (24.47 %) and constitutional symptoms (14.36 %). The difference in the number of DRP between treatment-naive (median 2) and pre-treated patients (7) was statistically significant (p = 0.031). The difference in the number of DRP between adjuvant (median 7.5) vs. palliative (7) and between different therapy regimens was not statistically significant. No correlation between patients’ age and the number of DRP was found. The level of concordance for categorisation of DRP was “good” using the PI-Doc® system (ê = 0.795, rate of uncodable DRP 15.63 %) vs. “very good” using the PCNE system (0.835, 12.50 %). For both categorisation systems additional codes were developed and tested. Introducing further codes led to a reduction of uncodable DRP (3.13 % vs. 4.17 %) but resulted in a lower rate of agreement (95.06 % to 82.81 % vs. 89.29 % to 78.26 %). CONCLUSIONS: Variety and frequency of the detected DRP demonstrate the need for pharmaceutical care in breast cancer patients especially for prevention and management of ADR. The use of both categorisation systems was found to be limited in this setting.