Objectives: Early diagnosis and treatment of Rheumatoid Arthritis (RA) and axial Spondylarthritis (axial SpA) can limit the impact of disease outcomes. This study evaluated the effectiveness of a referral program on the identification of patients with RA and axial SpA. Methods: This was an observational, prospective, randomized (by clusters) study conducted in Portugal to evaluate the impact of the implementation of a set of referral support actions (RSA). The study was divided in two sub-studies, the RA sub-study and the axial SpA sub-study. 28 participating primary care units were randomly (by clusters) assigned to RSA or control group (with no intervention). Both RSA and control groups identified and referred patients with suspected RA or axial SpA to the rheumatology unit of the reference hospital. The primary objective was to evaluate the correct diagnosis of RA or axial SpA cases confirmed by the rheumatologist of the reference hospital. Results: RA-Substudy: A total of 340 patients were recruited (144 in the RSA-exposed group; 196 in the control). RA diagnosis confirmation was 7.3% (95%CI, 2.1-12.5%) in RSA group versus 2.7% (95%CI, 0.0-5.7%) in control group RSA effect was positive but moderate (4.6%) and not statistically significant (95% CI, 0.0%-11.8%; p=0.222, adjusted for clustering effect). Rate of confirmed arthritis of any type was 16.9% (n=14/83) in the RSA group and 6.0% (n=5/83) in the control group. This difference was statistically significant and favorable to RSA group (OR=3.2; 95% CI 1.1-9.2; p=0.028). Axial SpA-Substudy: A total of 231 patients were recruited (108 in the RSA-exposed group; 123 in the control). Axial SpA diagnosis confirmation was 8.7% (95% CI, 2.1-15.4%) in RSA group versus 5.6% (95% CI, 0.0-11.73%) in control group. RSA effect was positive (3.1%) but not statistically significant (95% CI, -7.5- 12.9%; p=0.568, adjusted for clustering effect). Conclusions: This study showed a positive tendency for the RSA program, most relevantly on the diagnosis of patients with any type of arthritis in the RA sub-study. It is possible that a referral program more comprehensive than the one herein tested might improve early diagnosis of RA and SpA.