Psoriasis has a tremendous effect on health-related quality of life. Phototherapy and systemic treatments are used for patients with more debilitating (physically and emotionally) forms of the disease. These treatments can be extremely effective but can also have potentially significant adverse effects. The decision to undertake systemic treatment of psoriasis is a complex one that requires both experience and judgment. With the recent advent of new biologic systemic drugs for moderate to severe psoriasis, the need to clarify patient candidates for systemic therapy has become very important. Here, we present a diagnostic algorithm and a formal measure, the Koo-Menter Psoriasis Instrument (KMPI), to aid in identifying patients that would benefit from systemic therapy. In addition, the KMPI can be used to document and justify treatment decisions for health care payers. While the decision to undertake systemic treatment and the choice of specific treatment plan must ultimately be made mutually by the patient and physician, these tools are designed to provide information that will be valuable in these determinations.