Determination of urine cysteinyldopa excretion is the most sensitive chemical test for the detection of melanoma metastases and has been successfully applied during the Scandinavian winter, when sun irradiation is low. The value of this determination, under Australian conditions of greater sun irradiation, has been assessed by comparing the cysteinyldopa excretion of patients with that of normal subjects exposed to sunlight. Cysteinyldopa is an intermediate in the biosynthesis of the red‐brown pnaeomelanin.Of 20 patients without known secondary melanoma, the cysteinyldopa concentration of “spot” urines ranged from 0 to 190 (mean 48) μg/ml; of six known to have local metastases, 0 to 80 (mean 19) μg/ml; and of four known to have extensive metastases 80 to 1350 (mean 330) μg/ml.The effect of sun irradiation alone was assessed in nine healthy subjects followed one to 11 weeks before and after recorded periods of sun exposure. The cysteinyldopa concentrations of 24‐hour urines ranged from 40 to 3100 μg/ml. Increases occurred three to 10 days following sun exposure and were greatest following multiple small exposures in an individual with “Celtic” complexion.It is concluded that measurement of cysteinyldopa concentration would be of value in the follow‐up of melanoma patients in Australia only if patients could be persuaded to live under conditions free of all direc