To assess whether adopting a shared protocol between urologists and general practitioners (GPs) might change diagnostic procedures and referral patterns in the management of men with lower urinary tract symptoms (LUTS). Forty-five urological centres and 263 GPs in Italy participated in this prospective study. Procedures adopted by GPs for evaluating five consecutive patients (aged > or = 50 years) were compared before (phase 1) and after (phase 2) implementation of the shared protocol. An evidence-based diagnostic algorithm was developed and approved by participating urologists and presented to local GPs at a training session. Protocol modifications were allowed after discussion with GPs. Direct costs of diagnostic procedures carried out before and after implementing the protocol were calculated from the perspective of the national health service. In all, 903 patients were evaluable in phase 1 and 856 in phase 2. Implementation of the protocol did not change referral patterns, with about half the patients being managed entirely by GPs. The use of a digital rectal examination by GPs increased from 32% to 41%, use of transrectal and suprapubic ultrasonography decreased from 33% to 23% and 53% to 44%, respectively, (all P < 0.001) and use of the International Prostate Symptom Score increased from 4.5% to 23.1% (P < 0.001). Overall, protocol-recommended tests were used more frequently, while those not recommended decreased after implementing the protocol. However, overuse of the tests not recommended (i.e. urine culture and free/total prostate specific antigen ratio) remained high. The mean cost per patient of diagnostic procedures ordered by GPs decreased from Euros 71.82 to Euros 61.93, with Euros 9.9 saved for each patient. Our intervention failed to decrease the percentage of cases of LUTS being referred to specialists, but was moderately effective in inducing changes in the diagnostic management by GPs that were indicative of increased compliance with best-practice principles, and produced cost savings of 13.8%.