A significant fraction of cancer patients have occult disseminated tumors at the time of primary diagnosis, which usually progress to become clinically relevant lesions. Since the majority of cancer mortality is associated with metastatic disease, the ability to inhibit the growth of the secondary tumors would significantly reduce cancer-related morbidity and mortality. We have investigated whether caffeine, which has been shown to suppress tumor cell invasiveness and experimental metastasis, can suppress metastasis in a spontaneous transgene-induced mammary tumor model. Chronic exposure to caffeine prior to the appearance of palpable mammary tumors significantly reduced both tumor burden and metastatic colonization. However, when caffeine exposure began after the appearance of frank tumors, caffeine suppressed metastasis without changing primary tumor burden. The means by which caffeine suppressed metastatic activity may be associated with inhibition of malignant transformation of mammary epithelial cells, inhibition of conversion of dormant tumor cells to micrometastases, micrometastases to macrometastases, or inhibition of tumor cell adhesion and motility. Gene and protein expression patterns resulting from caffeine treatment showed that metastasis suppression may be associated with up-regulation the mRNA expression of multiple extracellular matrix genes, including Fbln1, Bgn, Sparc, Fbn1, Loxl1, Colla1, Col3a1, Col5a1, ColS5a2, ColSa3, Col6a1, Col6a2, and Col6a3. These data suggested that caffeine or other methyl xanthine derivatives may improve the clinical outcome in patients prior to and following the diagnosis of metastatic disease, and could potentially reduce the morbidity and mortality associated with disseminated tumors.