OBJECTIVES Budget impact analyses are an essential component in the economic evaluation of new drugs.  These analyses allow the healthcare payer to assess the likely impact of the drug on the payer’s budget, and to plan for short- and long- term resource allocation.  The objective of this study is to compare the company-predicted budget impact with the actual budget impact of high-cost drugs reimbursed in Ireland. METHODS All drugs submitted to the health service executive (HSE, healthcare payer in Ireland) for reimbursement under the high-tech drug scheme (a scheme used to administer high cost drugs) from 2009 to 2012 were included in the review.  Company estimates of the likely budget impact of the drug in 2013 were extracted from submissions and compared with actual expenditure in 2013 from the health service executive-primary care reimbursement service (HSE-PCRS).  Only drugs for which budget impact estimates were available and which were reimbursed by the HSE in 2013 were included in the analysis. RESULTS Ten drugs were included in the analysis, including six cancer drugs, two immunomodulators for multiple sclerosis and rheumatoid arthritis, and two orphan drugs for cystic fibrosis and idiopathic thrombocytopenic purpura.  The cumulative expenditure on these drugs in 2013 was €55.8 million compared with a predicted gross budget impact of €53.4 million, representing a €2.4 million underestimate in company submissions.  The most significant underestimate related to the drug for multiple sclerosis (€3.4 million) while the biggest overestimate related to the orphan drug for cystic fibrosis (€2.9 million). CONCLUSIONS Company submissions have been shown to both under- and over-estimate budget impact predictions.  It is important that budget impact estimates are as realistic as possible in order to effectively inform decisions on resource allocation or reimbursement.