The aim of this study was to assess risk factors for low back injury requiring medical advice or treatment among Iowa farmers. Although farmers are at risk for low back injury, few studies have addressed risk factors for farm work-related low back injury. We screened 6999 participants in the Iowa portion of the Agricultural Health Study to identify 49 male farmers who reported farm work-related low back injury requiring medical advice or treatment in the previous year. We compared them with 465 uninjured male farmer controls. Multivariable modeling identified four risk factors significantly associated with low back injury: age less than 45 years (OR = 3.32; 95% CI 1.75-6.20), doctor-diagnosed asthma (OR = 4.26; 95% CI 1.49-12.10), education beyond high school (OR = 2.12; 95% CI 1.13-3.90), and difficulty hearing normal conversation (even with a hearing aid, in the case of those using one) (OR = 1.98; 95% CI 1.02-3.80). Although hearing difficulty may be a general risk factor for occupational injury, asthma may be a more specific risk factor for low back injury. Future research to assess the risk factors, asthma and difficulty hearing, may be particularly important, since farmers are at increased risk for hearing loss, and farmers come into contact with many inhaled agents that can cause asthma.