It is shown that the subthreshold current and the threshold voltage of fully-depleted SOI (silicon-on-insulator) MOSFETs show a time-dependent behavior under certain front-gate and back-gate voltage conditions. An explanation for this behavior is provided. The SOI devices used in the study were MOSFETs on SIMOX (separation by implanted oxygen) wafers. The gate oxide thickness, silicon film thickness, and buried oxide thickness were approximately 118 angstrom, 700 angstrom, and 4000 angstrom respectively. The doping concentration was 2 × 1017 cm-3.