Multi-sensory stimulation provided in a Snoezelen room is being used increasingly for individuals with mental retardation and mental illness to facilitate relaxation, provide enjoyment, and inhibit behavioral challenges. We observed aggressive and self-injurious behavior in three groups of 15 individuals with severe or profound mental retardation and mental illness before, during, and after being in a Snoezelen room. All participants were receiving psychotropic medication for their mental illness and function-derived behavioral interventions for aggression, self-injury, or both. Using a repeated measures counterbalanced design, each group of participants was rotated through three experimental conditions: Activities of Daily Living (ADL) skills training, Snoezelen, and Vocational skills training. All other treatment and training activities specified in each individual's person-centered plan were continued during the 10-week observational period. Both aggression and self-injury were lowest when the individuals were in a Snoezelen room, followed by Vocational skills training and ADL skills training. The levels in the Snoezelen room were significantly lower than in both the other conditions for aggression but only in ADL skills training for self-injury. The difference in levels before and after Snoezelen were statistically significant with self-injury but not with aggression. The order of conditions showed no significant effect on either behavior. Snoezelen may provide an effective context for reducing the occurrence of self-injury and aggression.