At present, empirical evidence is scarce related to assertive behavior of family caregivers of patients with severe mental illness (SMI) and to the effects related to their deficits. The aim of this study was to examine the assertiveness of 140 family caregivers of 94 patients with SMI, and to examine the relationship between assertiveness and physical and psychological health. The Assertion Inventory (Gambrill and Richey, 1975), the Beck Anxiety Inventory, the Beck Depression Inventory II, the Perceived Stress Scale, and the Health Questionnaire SF-36 were used. 77.1% of the participants were assertive and there were no significant differences in base to socio-demographic variables. Assertiveness was related to anxiety, depression, stress, and general wellbeing as well as six (out of eight) areas assessed by the SF -36. The lack of assertiveness was a risk factor for all these variables. These findings support the importance of including assertiveness training in the treatment of the families of patients with SMI by staff at mental health units.