Abstract According to the Maastricht Treaty, a country seeking to join the European Monetary Union cannot have an inflation rate in excess of 1.5 per cent plus the average inflation rates in the three best performing EU countries. This inflation reference value is a non increasing function of the number of EU members. A counterfactual analysis of historical data shows that the effect of enlarging the EU from 15 to 27 countries was sizeable in 2002–04 and again from 2007. Monte Carlo simulations suggest that the enlargement of the EU from 15 to 27 members reduces the inflation reference value by 0.15–0.2 percentage points on average, but there is a considerable probability of a larger reduction at any given moment of time. The treatment of countries with negative inflation rates in the calculation of the reference value has a major impact on the results.