OBJECTIVES: A Budget Impact Analysis (BIA) analyses the financial consequence associated with the uptake of a new treatment option in the market; which can impact planning and forecasting and influence changes to pharmaceutical and health care budgets. Therefore, BIA forms a key consideration of the Dutch Health Care Insurance Board (ZiNL) when making a reimbursement decision regarding a new treatment intervention. The current analysis investigates consistent trends observed between the predicted budget versus the actual budget spent on reimbursed drugs in the Netherlands.  METHODS: Forecasted data specific to new drugs reimbursed under list 1B between 2009 and 2013 were extracted from drug reimbursement reports available from the ZiNL website. Actual data of the selected drugs were extracted from the Drug and medical devices Information Project database (GIP database). Per year and cumulative (between 1 and 5 years) data on total drug cost, total number of users and total cost per patient were compared between the forecasted values and the actual values.  RESULTS: In total 20 drugs were included in the analysis, of which 12 presented data for 3 years or more. Compared to the actual data, the expected total drug cost was overestimated for 14 drugs, total number of users was overestimated for 12 drugs and for 10 drugs the total cost per patient was overestimated. Total number of users was most accurately estimated, presenting a pooled cumulative overestimation of 5 times the actual number of users. Total drug cost was least accurately estimated (pooled cumulative overestimation of 13 times the actual total cost), since the total drug cost includes the uncertainty reflected in both the total number of users and total cost per patient.   CONCLUSIONS: The expected budget of 1B drugs in reimbursement submission reports generally overestimated the actual budget presented in the GIP database between 2009 and 2013.