In this prospective study we evaluated a new biochemical approach in which the plasma ratio of the melanin precursors L-dopa and L-tyrosine serves as a marker of metastatic dissemination in malignant melanoma. Control values (11.20 x 10(-5) +/- 2.92 x 10(-5)) were determined. The L-dopa/L-tyrosine ratio was evaluated in the plasma of 90 patients with malignant melanoma (stage I/II, n = 33; stage III, n = 33; stage IV, n = 24) classified according to the tumour/node/metastasis (pTNM) classification. A total of 106 samples were studied. Serial measurements were performed in eight stage III-IV patients. The L-dopa/L-tyrosine ratio was significantly elevated in melanoma patients with clinical stage III (15.23 x 10(-5) +/- 3.34 x 10(-5)) compared with stage I (10.88 x 10(-5) +/- 2.52 x 10(-5)). Stage IV patients showed a significant increase in the plasma L-dopa/L-tyrosine ratio (45.73 x 10(-5) +/- 61.75 x 10(-5)) compared with the other groups. The ratio was higher for those with two rather than one metastatic site and markedly higher for those with widespread metastases. The development of metastases was associated with an increase in plasma L-dopa, a decrease in plasma L-tyrosine and a significant increase in the plasma L-dopa/L-tyrosine ratio. These data suggest that the plasma L-dopa/L-tyrosine ratio reflects the tumour burden and correlates with the progression of malignant melanoma.