The purpose of our study is to prove that methodologies based on experimental functional analysis can be successfully adapted to the preschool classroom through a three-phase process of functional assessment, and be useful in designing effective treatments. In the first phase the disruptive classroom behavior of four preschool children was assessed using a functional assessment conducted through interviews and observations. Second, based on the information collected, hypotheses were formulated regarding the function of the problem behaviors. During the third phase a brief experimental functional analysis was used to verify our hypotheses. A treatment was then designed based on the factors involved in the problem behaviors. The treatment consisted in an adaptation of a classic DRO procedure applied functionally and resulted in a significant reduction of the problem behaviors which was sustained across the follow-up period. Our findings show that strategies combining an experimental functional analysis with a previous indirect observational functional assessment can contribute to the design of effective treatments at preschool level.