Questionnaires are essential tools for medical screening, but their role in monitoring workers at increased risk of occupational asthma (OA) remains indeterminate. Employees who were at a newly established wood products plant without previous exposure to methylene diphenyl diisocyanate (MDI) completed an initial questionnaire and from one to four follow-up questionnaires during a 2-year period. Onset of symptoms in 132 workers was assessed by exposure groups and modeled using generalized estimating equations. Onset of attacks of dyspnea with wheeze, attacks of dyspnea or cough at rest, and chest tightness were significantly associated with MDI exposure after controlling for age, smoking, and wood dust exposure. Onset of cough on most days was significantly related to smoking and dust. Onset of phlegm production was significantly related to both MDI and dust exposure. Onset of certain symptoms is significantly associated with MDI exposure. Early detection of MDI-associated health effects using a short screening questionnaire appears feasible.