Background Previously, we identified a series of T-cell epitopes from the major cat allergen Fel d1 and showed these were safe and well-tolerated when administered to cat allergic subjects. In the current study, we evaluated persistence of the treatment effect (tolerance) one year after start of dosing for two treatment regimens with the same cumulative dose. Methods Subjects underwent Baseline Challenge in an Environmental Exposure Chamber (EEC) on 4 consecutive days, consisting of 3-hour allergen exposures (Fel d1 50.19±3.70 ng/m3). Subjects scored four nasal and four ocular symptoms every 30 minutes each on a scale of 0-3. 202 subjects were randomised to placebo, 8x3nmol Cat Peptide Antigen Desensitisation (Cat-PAD) 2weeks(wk) apart or 4x6nmol Cat-PAD 4wk apart. Subjects re-attended the EEC for 4 consecutive days, of 3-hours, 18-22wk after the start of treatment. 50-54wk after commencement of treatment subjects were invited to participate in a blinded follow-on study without further dosing. 89 subjects were enrolled and had a further 4 consecutive days, of 3-hours in the EEC.