In the present paper I examine tax revenue projections in Germany over the period 1968 to 2012 with a focus on forecasting rationality. I show that tax revenue forecasts for themedium-term are upward biased. Overoptimistic revenue projections are particularly pronounced after the German reunification and reflect upward-biased GDP projections in this period. The predicted tax-GDP-ratio appears to be upward biased, as well. The forecasts are likely to overestimate tax revenues if the redicted tax-GDP-ratio exceeds its structural level of approximately 22 ½ percentage points. The results also indicate that forecast errors of short-term projections for the current year exhibit serial correlation. It is conceivable that the specific institutional setting can explain this non-rational behaviour to some extent.