We present a hybrid CUDA-MPI sorting algorithm that makes use of GPU clusters to sort large data sets. Our algorithm has two phases. In the first phase each node sorts a portion of the data on its GPU using a parallel bitonic sort. In the second phase the sorted subsequences are merged together in parallel using a reduction sorting network implemented in MPI across the cluster nodes. Performance results comparing our sorting algorithm to sequential quick sort yield speed-up values of up to 9.8 for sorting 4GB of data on a 32 node GPU cluster. We anticipate even better speed-up values using our algorithm on larger data sets and larger sized clusters.