Quality of life (QOL) assessments of women entering a UK randomised trial of adjuvant radiotherapy (START) were investigated to estimate the independent effects on QOL of age, time since surgery, type of breast surgery, chemotherapy and endocrine therapy. QOL was evaluated using the EORTC general cancer QOL scale (EORTC QLQ-C30), breast cancer module (BR23), the Body Image Scale (BIS) and the Hospital Anxiety and Depression Scale (HADS). Independent effects of age and clinical factors were tested using multiple regression analysis. A total of 2208 (mean age 56.9 years, range 26-87) consented to the QOL study prior to radiotherapy; 17.1% had undergone mastectomy (Mx) and the remainder had undergone a wide local excision (WLE). 33.3% had received adjuvant chemotherapy (CT) and 56.7% were taking endocrine therapy (ET). Age had significant effects on QOL with older and younger subgroups predicting poorer QOL for different domains. CT affected most QOL domains and resulted in worse body image, sexual functioning, breast and arm symptoms (<0.001). Mx was associated with greater body image concerns (p<0.001), and WLE with more arm symptoms (p=0.01). There were no effects of ET on QOL. Women <50 years (proxy pre-menopausal) had worse QOL in respect of anxiety, body image and breast symptoms but age and clinical factors had no effect on depression. Overall, QOL and mental health were favourable for most women about to start RT, but younger age and receiving CT were significant risk factors for poorer QOL, and so patients in these subgroups warrant further monitoring. Surgery had a limited impact and ET had no effect on QOL.