BACKGROUND AND OBJECTIVES: The occupational history of every adult patient with asthma provides information critical to the proper diagnosis and effective prevention of work-related asthma. This study determined the proportion of farm operators that reported an asthma attack while doing farm work that required the use of an inhaler or other medical treatment but who had not been told by a doctor, nurse, or other health professional that their asthma was related to work on the farm. METHODS: Asthma and asthma attack prevalences were estimated using data on a nationally representative sample of 12,278 active farm operators who participated in the 2006 Farm and Ranch Safety Survey. RESULTS: An estimated 4.9% of operators reported current asthma. Of these, an estimated 24.8% had been told that their asthma was related to work on the farm. Of those not so informed, 21.6% reported an asthma attack at work in the 12 months prior to the interview. CONCLUSIONS: A large proportion of farm operators who had not been told that their asthma was related to work on the farm experienced an asthma attack that occurred while doing farm work. These results suggest the need for improving clinicians' occupational health practices and clinician-patient communication.