The association between guttate psoriasis and infection with group A Streptococcus (GAS) has been well established in the medical literature. However, responses to treatments aimed at GAS eradication such as systemic antibiotics or tonsillectomy are inconsistent. Further complicating treatment recommendations for a disease with a suspected microbial trigger, the standard therapy for severe psoriasis is with systemic immunosuppressant medications. This case report illustrates the role of GAS as a trigger for acute onset severe psoriasis in a child whose skin disease initially worsened with a trial of methotrexate. An immune evaluation confirmed a co-existing selective antibody deficiency. Subsequent treatment with intravenous immune globulin dramatically improved his underlying immune function and decreased GAS infections. This improvement in overall immune function and decrease in GAS infections cleared his skin disease. An interval change in formulation to subcutaneous immune globulin was not as effective.