A personal 222Rn and gamma-ray detector has been developed. The detector precision is limited only by the Poisson counting error and has a lower limit of detection in this study of 4.5 kBq m-3 h (4 pCi L-1 day). The detector was used in a study of 52 homes in Illinois to measure the personal exposure vs. the simultaneous exposure on all levels of the home. The ratio of personal exposure to basement 222Rn concentration averaged 0.22, with a high degree of scatter (R2 = 31%). The ratio of personal exposure to first floor 222Rn concentration was 0.71, with good correlation (R2 = 85%). In the absence of personal monitoring data, the best estimate of personal exposure appears to be from measurements in the first-floor living space of the home.