Colloidal gold radionucleotide 198Au scanning can demonstrate the direction of regional lymphatic drainage in patients with primary malignant melanoma. Each of 32 patients with primary melanoma of the trunk received an intradermal injection of 0.1 mCi of 198Au colloid around the primary melanoma site. Imaging was accomplished with a tomographic scanner 24 hours following the injection. Twenty-seven of these patients underwent 36 regional lymph node dissections; nine had nodal metastases histologically demonstrated in the area (s) of nucleotide uptake. In clinical evaluation of up to 55 weeks, no patients demonstrated regional lymph node metastases in node groups which did not show preoperative gold uptake. Early evidence shows excellent correlation between the route of regional lymph node metastases and the direction of lymphatic flow to regional node groups, as demonstrated by gold colloid scanning. This procedure may be useful for evaluating patients with histologically defined high risk melanomas situated on the trunk in areas which drain to one or more lymph node regions.