OBJECTIVES: There are two preparations of Botulinum toxin type A - BOTOX and Dysport. Based on BNF prices, the dose ratio (units of Dysport : BOTOX) for cost equivalence is 4.2:1, however the two products are not interchangeable and there is no handy conversion factor. This study examined the dose requirements in a cohort of patients with dystonia who were stabilised first on Dysport then on BOTOX. The main objective was to evaluate the costs of each product in clinical practice. METHODS: Data was extracted retrospectively from case notes. Patients were included in the analysis if they had received each product for at least one year and had demonstrated a response to both. Those changed back to Dysport at any time during the 2-year period following the switch were excluded. Injections given in the 1-year period before the switch and between 1 and 2 years after the switch were included in the analysis. The mean dose of each toxin, the ratio and corresponding costs (from latest BNF) were calculated. RESULTS: Forty-two patients received 300 administrations. For spasmodic torticollis (36 patients) the mean doses of BOTOX and Dysport were 89 (range 53 to 120) and 397 (range 200 to 500) units respectively. The mean ratio for this indication (Dysport : BOTOX) was found to be 4.48:1 (95% confidence interval 4.22:1 to 4.73:1; range 2.6:1 to 6.3:1;). For this indication, the mean costs per administration were found to be £115 and £122 based on units used and £140 and £153 based on whole vials for BOTOX and Dysport respectively. Similar ratios were found for other types of dystonia giving an overall mean dose ratio of 4.56:1 (Dysport : BOTOX). CONCLUSION: This study is consistent with previous work that has shown that BOTOX is associated with lower costs than Dysport.