Between June 1985 and June 1987, developmental indoor radon reduction systems were installed in 40 houses in the Reading Prong region of eastern Pennsylvania. Most of these systems involved some form of active soil ventilation, although three involved heat recovery ventilators and two included carbon filters for removing radon from well water. The reduction in indoor radon concentrations achieved in each house was described in an earlier report. The purpose of the current study was to make follow-up alpha-track detector (ATD) measurements of radon concentrations in these houses, approximately one year after the last of the installations was completed, in order to determine how well the radon reduction performance of the systems was being maintained. The ATD measurements were made over a 3-month period during the winter (December 1987- March 1988), to assess system performance when cold weather would be giving the systems a significant challenge. These 1987-88 ATD results are compared with comparable post-mitigation ATD measurements made during the previous winter (1986-87), and with those made prior to the installation of the radon reduction system. Of the 34 houses where the radon mitigation system was in operation during the entire measurement period, the radon levels measured in 1988 compared well with those measured in 1987 (or any differences appeared explainable) in all but one house, indicating no significant degradation in system performance: in House 10, an exterior drain tile suction installation), levels had increased 50-70% from those measured in 1987, with no apparent reason. The well water radon removals achieved by the carbon adsorption units appeared not to have degraded since the previous water measurements. Of the 34 houses having operating active soil systems, two have experienced fan failure or need for fan maintenance in the 1-2 years that the fans have been in operation.