 Design Ten sets of multiple baseline measurement groups, 3 participants each n30 will be used to evaluate the effects of HBOT on the behavioral measures of adaptive mean length of utterance, MLU, aberrant, and stereotyped behaviors. The first participant in each multiple baseline will receive sessions of the behavioral measures for a minimum of one week prior to the implementation of HBOT. Subsequent participants cannot receive HBOT until an effect is observed as determined via visual inspection and statistical analysis of the data on prior participants or until prior participants have received 40 HBOT treatments, whatever occurs first. Subsequent to receiving 40 HBOT treatments, each participant will be given a break from HBOT for at least one week, during which time the schedule of behavioral measures will remain the same. If proficiency on the behavioral measures decreases when HBOT is terminated for one week, HBOT may be reinitiated as per the discretion of the physician and parent. 