A 145-acre potato field adjacent to Dorris, California, was treated with ethoprop (Mocap) to control nematodes. Ethoprop releases n-propyl mercaptan, a highly odorous and volatile gas, as a degradation product of the pesticide. An epidemiological investigation was undertaken by the California Department of Health Services because community residents sought medical attention for odor-related illness. Elevated health effects were found among those who reported smelling a strong odor (n-propyl mercaptan has a characteristic onion-like odor). In a logistic regression analysis, the most highly elevated 6-wk health effect incidence risks, expressed as odds ratios (ORs) adjusted for age, sex, and current cigarette smoking status, were for headache (OR = 5.08), diarrhea (OR = 3.80), runny nose (OR = 5.31), sore throat (OR = 3.58), burning/itching eyes (OR = 5.64), fever (OR = 3.59), hay fever attacks (OR = 3.50), and asthma attacks (OR = 6.0). Based upon these elevated health effects, it is recommended that human exposures to n-propyl mercaptan be minimized to the extent practicable.