For many people with brain injuries, mental retardation, or Alzheimer's disease, getting lost or disoriented is a common and distressing experience. At the University of Washington, computer scientist Henry Kautz's team has developed a system that uses cell phones to monitor users' whereabouts and help keep them on track. The phone, equipped with a GPS receiver that gauges its location, communicates wirelessly with a PC running novel artificial-intelligence software. Based on about three weeks of data, the software learns to predict daily behavior patterns, such as which bus a user takes. Then, if the system thinks the user is, say, getting off at the wrong bus stop, the phone sounds an alert and displays a text prompt on the screen -- including directions for getting home. Kautz's team plans to do tests next spring, together with University of Washington researchers in rehabilitation medicine. The software could be on the market within two years.