The extent of methyl tert-butyl ether (MTBE) contamination in private wells near gasoline stations, which lack the relative protection afforded many public waters supplies through zoning and routine testing, was examined. Samples were collected from 74 private wells near 21 randomly selected gasoline stations and from 21 control wells, one per facility. Two hypotheses are tested: (1) private wells downgradient and close (<0.5 mi) to gasoline stations (case wells) are more likely to have MTBE contamination than private wells upgradient and distant (>1.5 mi) (control wells); and (2) private wells near gasoline stations selling oxygenated gasoline are more likely to have MTBE contamination than private wells near gasoline stations selling conventional gasoline. Data on the concurrence of MTBE and other gasoline constituents are presented. RESULTS MTBE concentrations ranged from <1.0 micro/L (microg/L) to 61 microg/L, with a mean of 12.0 microg/L. MTBE contamination of > or =1 microg/L was detected more frequently in case wells (28%) than control wells (5%) (p = 0.015). MTBE contamination of > or =1 microg/L occurred more frequently in private wells near facilities selling oxygenated gasoline (38%) than private wells near facilities selling conventional gasoline (20%) (p = 0.11). Statistical significance may have been achieved with a larger sample size. Benzene (0.5 microg/L) was found concurrently with MTBE in only one well, which also had the highest concentration of MTBE.