A high‐speed energy correlation technique has been used to measure the time development of the width of the ultrashort pulses generated by a mode‐locked Nd+3glass laser. The laser was simultaneouslyQ‐switched and mode‐locked with a saturable absorber and produced a train of approximately 100 ultrashort pulses. It was found that the initial pulses in the train were of the shortest duration and that the pulse width increased from several picoseconds to about 15 picoseconds over the length of the train. The measurement technique and possible mechanisms for this behavior are discussed.