The way in which an electoral system influences the party system is a salient topic discussed in political science and social choice theory. Researches in post-communist countries have so far usually been analyses of the isolated cases of the elections. Here all free parliamentary elections in this region are examined. It allows measuring the total effect of the electoral system on the party system. It was found that the consequences of some electoral institutions are different than observed in old democracies: single-member districts with majority rule may cause considerable parliament fragmentation and the consequences of mixed systems are heterogeneous and depending on their inner mechanics. It was also found that the form of the party system depends to a greater degree on the properties of the political scene than on the electoral system.