Daytime plasma melatonin values were measured by radioimmune assay in 86 patients with breast cancer; 280 assays were done and compared with the clinical status of the patients. Patients in the advanced disease group had significantly higher levels than those in the adjuvant treatment group, and patients with progressive disease had significantly higher values than those in remission or with stable disease. No significant differences were found between different dominant metastatic disease sites. Multiple-regression tests showed a significant inverse correlation between survival and melatonin values.