Fluconazole is recommended in the prophylaxis of oropharyngeal candidiasis (OPC) in patients undergoing radiotherapy for head-neck tumours; however, the actual effectiveness of fluconazole in this setting remains unclear. Adult patients with cervico-cephalic carcinoma submitted to radical or adjuvant radiotherapy were randomized to 100 mg fluconazole (n = 138) or matched placebo (n = 132) oral suspension once daily from the sixth session of radiotherapy up to the end of treatment. The final analysis of the investigation showed a higher rate of the OPC outbreak-free survival in the fluconazole compared with placebo (P = 0.008 in the log-rank test). The mean time (95% CI) to OPC outbreak was 56 (53-59) days in the fluconazole group and 47 (43-51) days with placebo. The mean duration of radiotherapy was 43.5 and 39.9 days, respectively in the two groups (P = 0.027). Adverse effects were reported in 70.3% of patients in the fluconazole group and in 67.4% with placebo. The results showed prophylaxis with fluconazole given in irradiated patients with head-neck tumours significantly reduces the rate and the time to development of OPC compared with placebo.