Hyperbaric oxygen therapy (HBOT) has been used to treat individuals with autism. However, few studies of its effectiveness have been completed. The current study examined the effects of 40 HBOT sessions at 24% oxygen at 1.3 ATA on 11 topographies of directly observed behavior. Five replications of multiple baselines were completed across a total of 16 participants with autism spectrum disorders. No consistent effects were observed across any group or within any individual participant, demonstrating that HBOT was not an effective treatment for the participants in this study. This study represents the first relatively large-scale controlled study evaluating the effects of HBOT at the level of the individual participant, on a wide array of behaviors.