This paper presents a statistical analysis of anomalous, short duration, nighttime, electron temperature enhancements (NETEs) in the ionosphere over Millstone Hill (42N, 71W). These temperature enhancements are characterized by a several hundred degree increase in the topside electron temperature with a typical duration of two to four hours. The NETEs can occur anytime at night but most frequently between eighteen and twenty hours local time. Occasionally, two NETEs may be observed in the same night. The maximum temperature reached is often comparable to the daytime temperature. NETEs are most frequent in the winter, but do occur in the spring and fall. There are rare cases of the NETE occurring in the summer. There does not appear to be a clear dependency on either solar or geomagnetic activity. The NETEs association with winter suggests a relationship with conjugate photoelectrons which, if it exists, is not simple. The observed durations are too short to be attributed solely to conjugate photoelectron heating. ¿ American Geophysical Union 1994