Melanoma-inhibiting activity (MIA) is a 107 amino-acid protein secreted from melanoma cells and frequently detectable at high concentration in the serum of patients with advanced melanoma. Early studies suggested that MIA may be a useful serum tumor-marker for detection of recurrent or progressive disease. We evaluated the sensitivity of serum MIA levels in predicting the risk of relapse in patients with American Joint Committee on Cancer (AJCC) Stage II, III, and IV melanoma. MIA was measured by ELISA in serum from 39 patients with AJCC Stage II, III and IV disease at a single time-point 1 month to 5 years after they were rendered free of disease. Twenty-three of the 39 patients recurred, with a median follow-up of 4.5 months. Only four of the 23 patients who recurred had shown elevated MIA values (17% sensitivity). Of the 16 patients who remain free of disease (median follow-up 3.5 years, range 11 months to 6.3 years), one patient had an elevated MIA. There was no significant difference in the proportion of patients with elevated serum MIA between the patients who recurred and those who remained free of disease. In this series, serum MIA was not a sensitive marker for relapse in patients who were clinically free of disease after treatment.