In this paper, we consider the implementation of a thermal flow solver based on the lattice Boltzmann method (LBM) for graphics processing units (GPUs). We first describe the hybrid thermal LBM model implemented, and give a concise review of the CUDA technology. The specific issues that arise with LBM on GPUs are outlined. We propose an approach for efficient handling of the thermal part. Performance is close to optimum and is significantly better than the one of comparable CPU solvers. We validate our code by simulating the differentially heated cubic cavity (DHC). The computed results for steady flow patterns are in good agreement with previously published ones. Finally, we use our solver to study the phenomenology of transitional flows in the DHC.