Empirical data are presented to support the observation that nondifferential misclassification of subjects classified according to an ordinal scale can result in an artifactual nonmonotonic exposure-response curve. This phenomenon can be interpreted erroneously as an indication that an observed relationship is not causal. The occupational exposure of 40 subjects to extremely low-frequency magnetic fields was estimated on the basis of their job description while their "true" exposure was determined by direct dosimetry. A "true" risk was assumed to exist, and estimates of risk that could be obtained through hypothetical case-referent or cohort studies were calculated.