A randomised clinical trial was conducted to establish whether written instructions, in addition to verbal ones, significantly improve adherence to antibiotic treatment for acute sore throat in comparison with verbal instructions only. Patients were selected by consecutive sampling at seven primary healthcare surgeries. The pill count average was 87.4+/-25.2% and it was higher in the intervention group (93.7+/-24.5%) than in the control group (81.1+/-24.5%) (P < 0.05). Absolute risk reduction was 14% (95% confidence interval (CI), -3.77 to 26.56); relative risk reduction was 24.9% (95% CI, -11.04 to 58.28); the number needed to treat was 8.77. Written instructions, in addition to verbal ones, significantly improve compliance with antibiotic treatment in tonsillitis of acute sore throat in comparison with verbal instructions only.