Time-based lag sequential analyses were conducted on 23 topographies of challenging behavior shown by five young people with severe mental retardation across two settings. Potential behavioural functions were identified for 21 of the 23 behaviors. Responses classes, including two or more distinct behaviors, were identified for four of the five participants. Two participants showed evidence of two functionally distinct response classes; for one person each response class included both self-injurious and stereotypic behaviors. For 9 of the 11 behaviors for which data were available, the hypothesised function of behaviors was consistent across settings. For one person, data suggested that the function of one self-injurious behavior was contextually determined. Cross-validation with brief experimental analyses resulted in agreement on the general behavioral function for 12 of the 14 instances in which both approaches positively identified a potential function for a behavior. Implications for future research and practice are discussed.