Polyamine levels (putrescine, spermidine and spermine) in breast cancers (n = 54) were measured as a potential guide to prognosis. Values (expressed as nmol per 100 mg tumour) ranged from: 0.9 to 4.5 for putrescine, 4.2 to 29.8 for spermidine and 5.6 to 39.7 for spermine concentration. Increased intracellular polyamine levels were positively correlated with factors known adversely to affect survival after mastectomy, namely histological grade III and oestrogen-receptor negative status. Advanced T4 tumours and medullary-type carcinomas also contained high polyamine levels. Tumour size and node status did not affect polyamine levels in primary tumours. Tumours that recurred within 2 years of mastectomy had significantly higher levels of spermidine and spermine than those that did not. Breast cancer polyamine levels are a biological marker of tumour aggressiveness and can be used as a prognostic indicator of early tumour recurrence that is independent of node status.