Objectives: To determine the effectiveness of an intervention for pharmacy workers in improving their recognition and management of sexually transmitted disease (STD) syndromes. Methods: We randomly selected 14 districts (total population nearly 4 million) from the 24 districts of low socioeconomic status in Lima, Peru. We randomly assigned paired districts to receive training and support for management and prevention of STDs or a control intervention about management of diarrhoea. The STD intervention included interactive luncheon seminars on recognition and management of four STD syndromes (urethral discharge, vaginal discharge, genital ulcers, and pelvic inflammatory disease) and STD/HIV prevention counselling; monthly pharmacy visits by "prevention salespersons" who distributed materials that included "STD/HIV prevention packets" containing information, condoms, and cards given to patients for referral of their sex partners; and workshops for physicians on managing patients with STD syndromes referred from pharmacies. Standardized simulated patients visited pharmacies in intervention and control districts at one, three, and six months after training to assess outcomes. Findings: Standardized simulated patients reported significantly better recognition and management (appropriate antimicrobial regimens provided for discharge syndromes and referral to specially trained physicians for genital ulcers or pelvic inflammatory disease) by pharmacy workers of all four STD syndromes. They also reported significantly more frequent recommendations for use of condoms and treatment of partners at pharmacies in intervention districts than in control districts (by "intention-to-train" analyses, P≤0.05 for 47/48 primary outcome comparisons). Conclusion: Training was feasible and effectively improved pharmacy workers' practices.