The self-management behaviors of individuals with medical conditions that have an unknown etiology have not been studied. This study assesses the relationship between illness perceptions and various illness self-management behaviors in patients undergoing clinical genomic sequencing to identify a genetic cause for their condition. Hierarchical linear regression, Poisson linear regression, and logistic regression were used to assess the effect of illness perceptions (i.e., perceived consequences, timeline, personal control, treatment control, identity, concern, understanding, emotional impact, and causal beliefs as measured by the Brief Illness Perceptions Questionnaire) on healthcare use, prescription medication use, and doctor recommended supplement use, respectively (n = 200). Analyses revealed that (1) illness identity beliefs were positively associated with healthcare use (β = 0.20, p = 0.04), (2) both treatment control beliefs (B = 0.03, p = 0.02) and genetic causal beliefs (B = 0.17, p = 0.049) were positively associated with prescription medication use, and (3) both timeline beliefs (OR 1.23, p = 0.02) and emotional impact (OR 1.20, p = 0.02) were positively associated with doctor recommended supplement use. These findings can be used to inform the development of guidelines for treating patients who are seeking a genetic diagnosis for their illness.