We extend the relational approach to interprocedural analysis of sequential programs to concurrent programs composed of a fixed number of threads. In the relational approach, a sequential program is analyzed by computing summaries of procedures, and by propagating reachability information using these summaries. We generalize this approach to concurrent programs by computing for each thread procedure summaries that takes into account the parallel execution of the other threads. Technically, we define our analysis method by instrumenting the operational semantics of programs, then by abstracting the call-stacks of the concurrent threads, and last by abstracting the program environments in order to lead to an effective analysis that always terminates. This method allows to extend to concurrent programs existing relational interprocedural analysis (e.g., numerical variables analysis, shape analysis). We implemented it for programs with scalar variables, and we experiment several classical synchronisation protocols in order to illustrate the precision of our technique, but also to analyze the approximations it performs.