1. The hypothesis that a 3 degrees C elevation in temperature and doubled CO2 concentration would have no effect on the synchronization of winter moth egg hatch with budburst in oak was tested by comparing the separate and interactive effects of ambient and elevated (+ 3 degrees C) temperature and ambient and elevated (doubled to 340 p.p.m.) CO2 in eight experimental Solardomes. In addition, an outdoor control was compared with the ambient temperature/CO2 treatment combination. 2. Elevated temperature accelerated darkening (preceding egg hatch by about 5-10 days) and hatching of eggs developing off the trees; elevated CO2 had no effect. The same effects were observed in eggs developing on the trees. 3. Within treatments, date of egg hatch was the same on trees with early or late budburst. 4. Egg darkening and budburst were closely synchronized at both ambient and elevated temperatures. 5. Both eggs and trees required fewer cumulative heat units (day degrees > 4 degrees C), for hatching and budburst, respectively, at ambient than elevated temperatures. The requirements in the outdoor control treatment were similar to those in the ambient Solardome treatment. 6. Egg hatch between 10 and 25 degrees C, on a temperature gradient in the laboratory, required a constant number of heat units; fewer were required below 10 degrees C. 7. Elevated temperatures, in the Solardomes and the field, delayed adult emergence from the pupae. 8. The results suggest that a general increase in temperature with climatic change would not affect the closeness of the synchronization between egg hatch of winter moth and budburst of oak.