Forced spirometry is pivotal for diagnosis and management of respiratory diseases, but its use in primary care is suboptimal. The aim of the present study was to assess a web-based application aiming at fostering high-quality spirometry in primary care. This was a randomised controlled trial with 12 intervention primary care units (PCi) and six control units (PCc) studied over 12 months. All 34 naïve nurses (PCi and PCc) received identical training. The PCi units had access to educational material and remote expert support. Quality of spirometry and usability of the web application were assessed. We included 4,581 patients (3,383 PCi and 1,198 PCc). At baseline, quality was similar (PCi 71% and PCc 67% high-quality tests). During the study, PCi showed higher percentage (71.5%) of high-quality tests than PCc (59.5%) (p<0.0001). PCi had 73% more chance of high-quality performance than PCc. The web application was better for assessing quality of testing than the automatic feedback provided by the spirometer. Healthcare professionals' satisfaction and usability were high. The web-based remote support for primary care by specialists generated a sustained positive impact on quality of testing. The study expands the potential of primary care for diagnosis and management of patients with pulmonary diseases.