Total cathepsin B (catB) was determined by ELISA in 62 specimens of invasive ductal carcinoma of the breast. It was measured in microgram/g of tumour protein (microgram/gtp). The median catB was 91 micrograms/gtp, not varying significantly with T-stage or with age. It was higher in lymph-node negative (143 micrograms/gtp) than in lymph-node positive patients (49 micrograms/gtp) (P = 0.0005), in grade 3 (132 micrograms/gtp) than in grade 1 and 2 tumours (72 micrograms/gtp) (P = 0.07) and in hormone receptor-negative (155 micrograms/gtp) than in hormone receptor-positive tumours (72 micrograms/gtp) (P = 0.025). The recurrence-free survival (RFS) at 54 months for patients with tumours with catB < or = 23 micrograms/gtp was 22% and for catB > 23 micrograms/gtp, 68% (P = 0.0004). CatB > 23 micrograms/gtp did not significantly influence the RFS. Multivariate analysis showed that lymph nodes involvement (P = 0.003) and catB (P = 0.007) were independent prognostic factors.