Group purchasing cooperation can be beneficial for all parties involved. The gains are, however, not always fairly distributed amongst the participants. This puts a strain on the relationships within some forms of cooperation. Schotanus and Telgen (2005) have developed a classification model, the highway matrix, which makes a distinction between five forms of horizontal cooperations. This matrix distinguishes two factors to classify these cooperations, namely: the intensiveness for the members and number of different activities for the initiative. This matrix has been taken, because it is the most extensive classification tool that covers all possible forms of horizontal cooperations. This paper extends the highway matrix to incorporate the relationships between the parties involved, by adding a ‘symbiosis dimension’. This new classification tool, the extended highway matrix, assists purchasing cooperations, in the way how they should manage their relationship. More specifically, it provides information about the necessity for an allocation mechanism for gain distribution. It does not, however, provide information about the real distribution of gains. For more information about gain distribution the reader is referred to literature of Heijboer (2003) or Schotanus et al. (2006). This paper shows a re-analysis of 51 previously published case studies of group purchasing, which provides initial support for the new symbiosis dimension in the ‘extended highway matrix’. Based on the analyzed cases it can be concluded that some forms of group purchasing have no need for a special allocation mechanism for gain distribution, whereas others do in order to run the group successfully.