Histogram reweighting offers a powerful means for extending the information obtained from Monte Carlo simulations to different thermodynamic states. With only a few simulations the entire vapor-liquid coexistence region of a pure fluid can be investigated. We have developed a histogram reweighting method for the isothermal-isobaric (NPT) ensemble and compared it to a method in the grand-canonical ensemble. Results are presented for a Lennard-Jones fluid and a fully flexible SPC water model with and without long-range interactions. For the Lennard-Jones fluid both ensembles give equivalent coexistent densities. The agreement for the screened water potential (i.e., no long-range interactions) is also satisfactory. The full water model was analyzed via reweighting of NPT-ensemble histograms and was found to agree with data obtained for the SPC water model in the Gibbs ensemble. The NPT histogram reweighting method is demonstrated to be an effective tool for examining phase equilibria and may have great utility in situations for which grand-canonical simulations are difficult. (C) 1998 Elsevier Science B.V. All rights reserved.