From 1973 through 1979, inguinal node biopsy was performed to stage the disease process in 179 clinical Stage I patients with malignant melanoma of the lower extremity, who were all treated by hyperthermic regional perfusion as well. Of the 179 tumors, 12% were intermediate risk (0.75-1.44 mm) and 88% were high risk (greater than or equal to 1.5 mm); all had a Clark level of IV or V. The Rosenmüller node at the caudal margin of the saphenous hiatus was elected for inguinal node biopsy. This biopsy supplies a fair amount of information about the entire inguinal node region: a malignant node was found in 16 patients (9%); no other metastatic nodes were found in 11 (73%) of 15 subsequent therapeutic node dissections; the 16th had metastatic parailiac nodes as well. Two patients of the remaining 163 had only metastatic parailiac nodes, without metastatic inguinal nodes. Of the remaining 161 histologic Stage I patients, 23 (14%) developed inguinal node metastases in the course of the follow-up. In 17 (74%) these metastases occurred within 2 years of perfusion. Ten of the 23 showed simultaneous general metastases. The vast majority of the inguinal node metastases developed in patients with a tumor greater than or equal to 5 mm. The 5-year survival was 81%, i.e. 84% in females versus 69% in males, the difference being significant (P less than 0.01). A tumor thickness greater than or equal to 5 mm implied a significantly less favorable prognosis as to development of inguinal node metastases associated with general metastases than a tumor thickness less than 5 mm. The benefit of the inguinal node biopsy was related to the difference in 5-year survival between the group with inguinal node metastases at perfusion (69%) and the group who developed inguinal node metastases during the follow-up (24%). The difference was great (45%) but statistically not significant. The data seem to warrant the conclusion that, after perfusion therapy, inguinal node biopsy is sufficient to stage the disease process at a tumor thickness less than or equal to 5 mm. Given a tumor thickness less than or equal to 5 mm, elective groin dissection might improve the chance of survival.