We determined the serum concentration of interleukin-8 (IL-8) in patients with chronic sarcoidosis and assessed its value as a marker of disease activity. The serum concentrations of IL-8 and tumor necrosis factor-alpha (TNF-alpha) were determined by enzyme-linked immunosorbent assays. Serum angiotensin-converting enzyme (ACE) activity was measured by a colorimetric assay in 16 patients with chronic sarcoidosis. Thirteen of the 16 patients with chronic sarcoidosis showed a serum IL-8 concentration higher than the normal range. The mean (+/- S. E.) serum IL-8 concentration in patients with active sarcoidosis (n = 6) was 18.7 +/- 1.3 pg/mL and that in patients with inactive sarcoidosis (n = 10) was 8.0 +/- 1.1 pg/mL; the difference was statistically significant (p = 0.002). An overall positive correlation between serum IL-8 concentration and serum ACE activity was apparent; however, only four of the 16 patients with sarcoidosis had a serum ACE activity greater than the normal range. TNF-alpha was detected in the serum of only two patients. The serum concentration of IL-8 may prove a useful marker for monitoring disease activity in sarcoidosis, especially in the chronic stage.