This paper documents the validation of a quality of life scale (QOL) designed to assess the impact of arm morbidity on patients following breast cancer surgery. A four item arm subscale was developed to supplement a multi-dimensional, validated breast cancer QOL tool, the functional assessment of cancer therapy (FACT-B.) The new questionnaire, the FACT-B + 4, was validated on 279 women participating in a trial of sentinel node guided axillary therapy and 29 women attending a lymphoedema clinic. The subscale demonstrated good internal consistency (alpha co-efficient = 0.62 to 0.88) and stability (test-retest reliability = 0.97). Lymphoedema patients reported significantly greater arm problems than a matched sample of pre-operative trial participants. The lymphoedema group also scored lower than trial patients on the FACT-B + 4 indicating a poorer quality of life (p < 0.05). A subset of 66 trial patients who had completed three consecutive assessments was used to evaluate the sensitivity of the questionnaire to change over time. Scores on the FACT-B + 4 were found to decline significantly between the pre-operative assessment and post-operative assessment at 1 month. Arm problems significantly increased during this period. FACT-B + 4 score increased again from 1 month to 12 weeks post-surgery and symptoms reduced, as the extent of arm morbidity resolved. The FACT-B + 4 appears to be psychometrically robust and sensitive to patient rehabilitation, making it suitable for use in longitudinal surgical trials. Given the dearth of existing scales available to measure arm morbidity, we hope this new tool will prove useful to researchers.