This cross-sectional study aimed to investigate the effect of supraclavicular fossa (SCF) radiotherapy volumes as well as patient characteristics and nodal pathology on the development of lymphoedema. Ninety-one women who had received SCF nodal radiotherapy after axillary dissection were evaluated. Lymphoedema was defined by two measurements: limb volume difference 200 mL, or circumference difference 10 cm proximal or distal to the olecranon >2 cm. On univariate analysis, the addition of axillary to SCF radiotherapy, increasing width of the SCF field, increasing age, presence of extracapsular extension of nodal involvement and use of hormone treatment was associated with lymphoedema by either one or both definitions. For both definitions of lymphoedema, on multivariate analysis, increasing nodal radiotherapy volume remained significant (P = 0.02 to 0.007), as did increased age (P = 0.05 to 0.001). We conclude that conventionally fractionated SCF radiotherapy limited laterally by the coracoid process has a lymphoedema risk similar to that expected from axillary dissection alone and a lower risk than wider SCF fields with or without an axillary boost.