In the modern hospital environment, many health care workers are exposed to hazardous substances. Among these hazards are respiratory sensitizers, irritants, and infectious agents. A previous cross-sectional study of Rhode Island respiratory therapists reported an excess risk of asthma after entry into that profession. Before the results of that study were published, we conducted a confirmatory mailed questionnaire survey of 2,086 Massachusetts respiratory therapists and 2,030 physical therapists and physical therapy assistants. Neither the survey questionnaire nor the accompanying cover letter revealed the focus of our investigation. A history of physician-diagnosed asthma was reported by 16% of respiratory therapists and 8% of control subjects. When analysis was restricted to those who developed asthma after entry into their profession, respiratory therapists still had a significant excess, 7.4 versus 2.8%. The odds ratio for respiratory therapy was 2.5 (95% Cl, 1.6 to 3.3) after adjustment for age, family history, atopic history, smoking, and gender. These results confirm the previous report of excess risk of asthma among respiratory therapists. This excess risk develops after entry into the profession and does not appear to be explained by bias or confounding. Efforts should be directed to identifying potential agents responsible for this form of occupational asthma.