Squamous cell carcinoma of the head and neck produces a prostaglandin, PGE2, a potent inhibitor of cellular immune responses. We tested the effects of prostaglandin synthetase inhibition on the infiltration of squamous cell carcinoma of the head and neck with host lymphocytes. Tumor tissue samples were obtained from six patients (age range, 51 to 72 years) who presented with squamous cell carcinoma of the head and neck before and after 14 days of treatment with indomethacin (50 mg administered orally three times a day). Tumor-infiltrating immune cells were assayed in frozen tissue samples by indirect immunofluorescence. An eightfold increase in CD2+ lymphocytes compared with pretreatment tissue was observed. The number of CD4 and CD8 lymphocytes increased similarly. CD57 lymphocytes increased 15-fold and CD11b cells increased 11-fold. No infiltrating B-cell populations were evident. Double-labeling studies revealed that the mononuclear cells were located outside blood vessel walls, indicating that they had infiltrated the tumor parenchyma. Our findings demonstrate that the administration of indomethacin is associated with the increased immune cell infiltration of squamous cell carcinoma of the head and neck. This suggests that inhibition of PGE2 synthesis as it occurs in the tumor and or systemically may contribute to the homing of mononuclear cells to the tumor. These data suggest a mechanism to account for the clinical response to indomethacin previously reported in squamous cell carcinoma.