Mixed findings surround the notion people with a left hand preference report more autoimmune, mental health, and developmental disorders (e.g., Cazzoli & Chechlacz, 2017; Geschwind & Behan, 1982). How handedness is measured in the literature varies and often does not consider degree of handedness, which is thought to be more representative of neural upper limb lateralization (Luders et al., 2010). Cognitive functioning has shown hemisphere dominance in the brain and performance in these areas are associated with health conditions that are in-line with testosterone theories of handedness. The purpose of this study was to further investigate relationships among handedness, health, and performance on right and bilateral hemisphere dominate tasks. Online assessments completed by 292 participants included; the Waterloo Handedness Questionnaire (Steenhuis et al., 1990), a health questionnaire, a mental rotation task, and the N-back task (Kirchner, 1958). Findings indicated a number of complex interactions. Autoimmune disorders (p=.028), anxiety (p=.024), thyroid disorders (p=.023), speech problems (p=.026), and asthma (p=.017) interacted with handedness on performance on the mental rotation task. Allergies (p=.049), substance abuse (p=.038), and gastrointestinal disorders (p=.016) interacted with handedness on performance on the N-back task. Support was found for measuring handedness as a degree: left and right handers performed better than inconsistent handers. Inconsistent handers who did not report certain health aliments performed better than inconsistent handers who did.