Background: So far, no study has tested supportive-expressive group therapy (SEGT) in cancer patients with an established psychiatric diagnosis. The aim of this 6-month follow-up study was to evaluate breast cancer patients with an ICD-10 diagnosis of affective syndromes participating in SEGT and a group of breast cancer patients with no ICD-10 diagnosis. Methods: A total of 214 patients were examined in the screening phase (T0) using the ICD-10, the Brief Symptom Inventory (BSI), the Mini-Mental Adjustment-to-Cancer Scale (Mini-MAC), the Multidimensional Scale of Perceived Social Support, the Openness Scale and the Cancer Worries Inventory (CWI). Those with an ICD-10 diagnosis of affective syndromes received 16–24 sessions of SEGT (90-min sessions, once a week), while those with no ICD-10 diagnosis were followed up. A second assessment for both sam- ples took place 6 months later (T1). Results: Seventy-eight (36.4%) patients were positive for an ICD-10 diagnosis of affective syndromes at T0, while 127 (59.4%) did not meet any ICD-10 diagnosis. Among the former, 54 patients participated in the SEGT. At T1, significant differences were observed in all the dimensions of the BSI, hopelessness and anxious preoccupation subscales of the Mini-MAC, the Openness Scale and the CWI. No variable at T0 was a predictor of BSI distress as measured at T1. Among those with no ICD-10 diagnosis at T0, 8.2% were positive for affective disorders at the 6-month follow-up. Conclusions: This study suggested that SEGT is effective for breast cancer patients with affective disorders, and indicates the need for prospective evaluations in order to identify those who may develop psychopathology over time.