The most common disorder of the facial nerve is acute idiopathic facial paralysis or Bell's palsy and there may be significant morbidity or incomplete recovery associated with severe cases. Although the cause remains unknown, recent evidence suggests a possible association with Herpes simplex virus (HSV) infection. To test this hypothesis clinically four clinical trials have used aciclovir, an antiviral agent, either alone or in combination with corticosteroids to treat Bell's palsy. To assess the efficacy of aciclovir for treating Bell's palsy and to evaluate any adverse effects of the drug treatment. Search of the Cochrane Neuromuscular Disease Group register, MEDLINE, EMBASE and LILACS databases for randomised trials. We also contacted authors of identified trials. Randomised or quasi-randomised trials of aciclovir therapy, alone or in combination with any other drug, in patients with Bell's palsy. We identified four randomised trials. One author extracted the data and the other checked them. We wrote to all the authors of the trials identified. No additional data were obtained. Only two studies met our inclusion criteria and provided results from 200 patients. One study evaluated aciclovir with corticosteroid versus steroid alone and the other study evaluated aciclovir alone versus corticosteroid. PRIMARY OUTCOME Proportion of patients with incomplete recovery after one year: These data were not available. However an analysis was performed on data reported at the end of the study period, three (De Diego 1998) or four months (Adour 1996) after the start of treatment. The results from the Adour study significantly favoured the treatment group whilst the De Diego study significantly favoured the control group. Adverse events: This analysis was not performed as the data were not reported. Complete facial paralysis six months after start of treatment: Only one patient had complete paralysis upon entering only one of the studies (De Diego 1998). This patient was assigned to the control group and the level of recovery attained was not reported. Motor synkinesis/Crocodile tears one year after start of treatment: Data were available up to a maximum of four months after onset of paralysis. One study reported a significant difference between the treatment groups in favour of the aciclovir group and the other demonstrated an inconclusive result. More data are needed from a large multicentre randomised controlled and blinded study with at least 12 months' follow up before a definitive recommendation can be made regarding the routine use of aciclovir in Bell's palsy.