In a series of 73 patients with mucoid breast carcinomas treated at Guy's Hospital between 1973 and 1989, 24 (33%) patients had pure mucoid lesions and 49 (67%) had mixed mucoid carcinomas. The patients with pure mucoid cancers had significantly smaller tumours and, among those in whom an axillary dissection was performed, mixed mucoid cancers were more likely to be associated with axillary nodal metastases (46% vs 14%). After long-term follow-up of 64 patients, both relapse-free and overall survival were significantly better for those with pure mucoid carcinomas, for whom the 10-year actuarial overall survival was 100%. The overall proportion of the tumour that was mucoid was also positively associated with a more favourable prognosis in patients with mixed tumours. With such a good prognosis, patients with pure mucoid carcinomas may not require systemic adjuvant therapy after adequate primary treatment.