Interoffice fiber-optic networks are already a multi-million dollar business for U S WEST Communications, with rapid expansion anticipated for the rest of the decade. U S WEST's operations research modeling group developed a mixed-integer programming model to help plan the least-cost expansion of interoffice fiber-optic networks in major metropolitan areas throughout the 14 midwestern states in U S WEST's territory. The model augments previous telecommunications planning tools by developing explicit optimal expansion plans for multi-year planning horizons. It was tested successfully in 1991 on network and demographic data from Colorado Springs, Colorado. An outgrowth of the model is now being used to design and plan the growth of fiber-optical ring networks in Seattle, Denver, Minneapolis, and other major metropolitan areas throughout U S WEST's region.