More is known about how to reduce challenging behavior with functional communication training (FCT) than how to mitigate its resurgence during or following a course of treatment. Research suggests reinforcing mand variability during FCT may mitigate the resurgence of challenging behavior, but validated procedures for reinforcing mand variability are limited and poorly understood. Lag schedules can reinforce variability in verbal behavior such as manding in individuals with autism, but studies have been largely limited to nonvocal mand modalities. Therefore, in the current study, we further evaluated the effects of FCT with lag schedules on vocal mand variability and challenging behavior in children with autism. The results suggest lag schedules alone or in combination with response prompt-fading strategies during FCT can increase mand variability and expand mand response classes but may fail to produce clinically significant reductions in challenging behavior.