The Peruvian Government has embarked on a major rehabilitation program for its water and wastewater system. SEDAPAL is the governmental agency in charge of the operation and maintenance of the water and sewerage system in Lima, Peru. The agency provides services to approximately seven million people. Due to the magnitude of the system and the shortage of adequate personnel and equipment, the system has not received optimal maintenance, resulting in a high percentage of unaccounted-for water, low pressure conditions, and less than perfect water quality. The program study area of approximately of 21.6 square miles is located within Lima's metropolitan area. A total of ten districts, similar in size to cities in other countries, are included in the study area. Part of the rehabilitation program includes the preparation of a computer model of the water distribution system. Specific sections of the system were selected as being representative of the system as a whole to be used for detailed field analysis. The primary purpose of the field analysis was to determine representative pipeline coefficients of friction to use in the model and therefore, areas with one specific type of pipe material and those known to include advanced pipe age conditions were selected for investigation in the field. The model incorporates all the physical characteristics of the distribution system, including pipeline lengths, diameters, materials of construction, elevations, and coefficients of friction (roughness coefficients). For each area tested in the field, measurements were made for the total flow consumption into the area and for pressure values at specific points distributed throughout the area. The results of the field measurements were compared to the results of the system model. This comparison assisted in the determination of the theoretical coefficient of friction for each pipe material, and also assisted in performing necessary adjustments to the amount of flow consumption based on high water losses and unexpected consumption conditions. This paper focuses on the interactive approach between field measurements and network model results for water distribution systems that experience high rates of water loss and that have minimal amounts of historical information available. Includes tables, figures.