In 1971, sludge wastes contaminated with 2,3,7,8-tetrachlorodibenzop-dioxin were mixed with waste oil and sprayed on a dirt road at the Quail Run Mobile Home Park in Gray Summit, Mo. We performed a comprehensive examination of 154 exposed and 155 unexposed persons in the area. There were no consistent differences between the two groups on medical history, physical examination, serum and urinary chemistry studies, and neurologic tests. Results of liver function tests suggested possible subclinical effects. The exposed group had an increased frequency of anergy (11.8% vs 1.1%) and relative anergy (35.3% vs 11.8%). The exposed group also had non-statistically significant increased frequencies of abnormal T-cell subset test results (10.4% vs 6.8%), a T4/T8 ratio of less than 1.0 (8.1% vs 6.4%), and an abnormality in the functional T-cell test results (12.6% vs 8.5%). These findings suggest that long-term exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin is associated with depressed cell-mediated immunity, although the effects have not resulted in an excess of clinical illness in the exposed group. Further studies are indicated to elucidate the pathophysiology and clinical significance of these immunologic findings.