Estimates of transistor performance at millimeter-wave frequencies are generally based on extrapolation of microwave gain measurements at a gain roll-off of 6 dB per frequency octave. In this paper we show that a complex-conjugate pole pair leads to a 12-dB/octave gain roll-off in the millimeter-wave region. As a result the actualf_{\max}of millimeter-wave transistors can be considerably less than that determined using extrapolation of gain measurements.