The concentrations of trimethoprim (TMP), sulfadiazine (SD) and acetyl-SD in serum and maxillary sinus secretions of 10 patients with acute maxillary sinusitis were measured at the steady-state of continuous b.i.d. treatment with 160 mg of TMP + 500 mg of SD. The samples were collected 2–4 h after the preceding dose of TMP/SD. The mean TMP concentrations were 2.97 ± 0.35 µg/ml (mean + SE) and 3.85 ± 1.12 µg/ml in serum and sinus secretion, respectively. The average sinus secretion/serum ratio was 1.33. The mean SD levels in serum and sinus secretion were 41.9 ± 4.5 and 6.64 ± 1.40 µg/ml, respectively, and the average sinus secretion/serum ratio was 0.20. Most patients (7/10) had detectable amounts of acetyl-SD in sinus secretions (mean 1.92 ± 0.87 µg/ml). The corresponding concentrations in serum were 4.56 ± 0.47 µg/ml.