Objective: We aimed to evaluate the efficacy of and immunologic changes caused by subcutaneous immunotherapy (SCIT) in patients with allergy to cat and dog. Methods: Patients presenting rhinitis and/or asthma with allergy to cat or dog from a previous safety study were included. All had specific IgE to cat and/or dog. Using an infusion pump (IP), SCIT maintenance dose was administered over one 4-hour session, followed by monthly administration over 6 months. Data were gathered on clinical outcomes, pulmonary function, FeNO, rhinitis and asthma symptoms, quality of life (QoL), asthma control test (ACT), and symptom visual analog scale (VAS) at baseline and then at 1, 3, and 6 months. Specific IgE and IgG antibody responses to different cat and dog allergens were determined. Results: Sixty-one patients having a mean age of 35.6 ± 9.7 years were included, 40 of whom underwent cat SCIT. A significant improvement was observed in rhinitis and asthma symptoms and in QoL, use of medication, VAS, and ACT at 1 month; these improvements persisted at month 6. Clinical improvement with cat extract was significantly higher than with dog. An increase of >0.9 in ESPRINT-15 health-related quality of life in allergic rhinitis) was observed in 49.09% of patients, and 58.18% showed an increase of >0.5 in AQLQ(asthma quality of life questionnaire), at month 6, both differences indicating the minimal important difference. A significant increase was observed in specific IgG and IgE to different allergens at 3 and/or 6 months. Conclusions: Ultrarush SCIT with cat and dog extracts has substantial clinical value in many patients.