We sought to determine mutuality and specificity in rates of mental disorders between advanced cancer patients and their caregivers. Data from 168 non-genetically related patient-caregiver dyads participating in the multi-site Coping with Cancer (CWC) study were included in this analysis. Multivariate logistic regression analyses were conducted to examine associations between diagnoses of a psychiatric disorder in patients with diagnoses of psychiatric disorders in caregivers, and vice versa, controlling for confounders. When patients met criteria for any psychiatric diagnosis, then caregivers were 7.9 times (P < 0.0001) more likely to meet criteria for any psychiatric diagnosis, and vice versa. Caregiver Panic Disorder (PD) diagnosis was associated with patient Generalized Anxiety Disorder (GAD). Patient GAD was also associated with caregiver PD. Finally, patient PD was associated with caregiver GAD and caregiver Post-Traumatic Stress Disorder (PTSD). To our knowledge, this is the first study that demonstrates the mutuality of psychiatric disorders in both advanced cancer patients and their informal caregivers. Specifically, the presence of anxiety disorders in one partner (either caregiver or patient) was associated with a greater likelihood of anxiety disorders in the other. Results suggest that psychiatric distress should be assessed in both patients and their caregivers, and that mental illness in one should raise concern about the possibility of a psychiatric disorder in the other. Results also suggest that targeted interventions to address shared fears and concerns of patients and caregivers might reduce anxiety in the end phases of the patient's illness.