Cathepsin D (cath-D) is an aspartyl lysosomal protease expressed in all tissues. Most metastatic breast cancer cell lines, unlike normal cells, secrete high levels of pro-cath-D. This abnormal secretion is due to both overexpression of the cath-D gene and to an altered processing of the precursor protein. Cath-D gene transcription is increased by estrogen and growth factors in estrogen-receptor-positive breast cancer cells and by an unknown mechanism in estrogen-receptor-negative cells. A large number of independent clinical studies associated high cath-D concentrations in the cytosol of primary breast cancers with increased risk of subsequent metastasis. The amino acid sequence of cath-D analyzed in two breast cancer cell lines is normal, but glycosylation appears to be different with more acidic isoforms. To assess the potential role of this protease in cancer metastasis, we transfected a human cDNA cath-D expression vector in 3Y1-Ad12 embryonic rat tumorigenic cells which did not secrete the proenzyme. A moderate overexpression of human cath-D was sufficient to increase the metastatic potential of these cells in nude mice. The mechanism of cath-D-induced metastasis seems to require maturation of the proenzyme, in endosomes and in large acidic compartments identified as phagosomes. Rather than increase cancer cell escape from the primary tumor through basement membrane degradation as proposed for neutral proteinases, cath-D appears to facilitate cell growth at distant sites. The mechanism of this indirect mitogenic effect is discussed from results obtained in different models. Different cath-D substrates (growth inhibitors, precursors of growth factors, etc.) are proposed to mediate this activity.