Summary. The paper presents estimation results on the size and loyalty of sport teams’ supporter groups in professional German football. Based on a novel two-stage estimation procedure, we find clear evidence for heterogeneity across teams. In the first stage, a random-utility model for a representative consumer is modelled and fitted to more than 1700 matches over the seasons 1996–2001. In the second step, attendance probabilities are predicted for the seasons 2002–2003 to estimate group sizes. A team's group size is positively correlated with its memberships (.61; p<0.01), fan clubs (.59; p<0.01) and merchandizing revenues (.49; p<0.05). Noteworthy is that no similar correlations can be found for a team's home town population which has been the standard measure for market size in applied work so far.