Functional analyses of problem behavior for 4 young boys with developmental delays showed that problem behaviors were maintained by both negative and positive reinforcement. Functional communication training was conducted with one set of training stimuli in which the child's mother presented a work task in the family living room. Pretreatment probes were conducted with up to 10 sets of generalization stimuli that varied across tasks, settings, and people. Post treatment probes were conducted across the same generalization stimulus sets. Results for four dependent variables (problem behavior, task completion, manding, and social interactions) indicated that (a) problem behavior occurred across some but not all generalization stimulus sets during pretreatment; (b) following training, reductions of 90% or greater were observed within the context of the training stimuli; (c) reductions in problem behavior were observed for 70% of the post treatment generalization stimulus sets, but the reductions were not as large (M = 61%) as those observed with the training stimuli; and (d) the most consistent change observed during post treatment probes of generalization stimulus sets was increased task completion. Parents of all participants completed a behavior rating form and reported reductions in problem behavior across activities. (Contains 2 tables and 11 figures.)