Recent advances in horizontal drilling and completion technology have led to the successful completion of multilateral horizontal wells (wells with two or more laterals penetrating separate zones that produce through a single wellbore to the surface). Evaluating performance of these commingled horizontal wells poses unique problems that have led to development of a practical procedure for that purpose. This paper demonstrates a practical procedure used in evaluating the performance of three lateral (trilateral) horizontal wells. The procedure, coupling the analysis of pressure-buildup and rate/time data, involves use of a consistent mathematical model to generate pressure and rate responses for pressure-test interpretation and performance prediction. The procedure is demonstrated for several new trilateral wells drilled in the Dos Cuadras field, offshore California. Results from field tests indicate that the procedure is efficient in evaluating the performance of these wells. Reservoir and well parameters are obtained and the contribution of all laterals is confirmed from pressure-test interpretation. The computed model parameters are then used in a rate history match and accurate longterm performance predictions are made. Deconvolution is shown to improve the history-match process. In addition, the performance evaluation of these trilateral wells is compared with previously drilled single-lateral horizontal wells in the same field. The comparison confirms that the new trilateral horizontal well technology is economically viable, allowing reserves to be developed for approximately two-thirds the cost of three single-lateral horizontal wells.