The effect of a therapeutic intervention in a clinical trial may be obscured by heterogeneity in the study subjects. This paper examines the results of a randomized clinical trial to evaluate the effects of 6 months of sensory integration therapy, perceptual-motor treatment, and no program (control) on learning disabled children with sensory integrative dysfunction. Contrary to expectations, Polatajko, Law, Miller, Schaffer, and Macnab (1991) found no significant differences between the groups on motor performance. However, overall, the children made motor gains; indeed, some children made very large gains. Others did not make gains or deteriorated. Change scores indicated that approximately half of the children improved more than can be expected by maturation alone while half did not. The heterogeneity of the response of individual children to treatment appeared to have led to an overall non-significant result. The heterogeneity was examined using a backward elimination regression procedure. The importance of this heterogeneity for assigning children to programs and designing outcome studies in occupational therapy is discussed.