Legislatively mandated programs for early intervention on behalf of handicapped infants often stipulate the inclusion of physical therapy as a major component of treatment for cerebral palsy. To evaluate the effects of physical therapy, we randomly assigned 48 infants (12 to 19 months of age) with mild to severe spastic diplegia to receive either 12 months of physical therapy (Group A) or 6 months of physical therapy preceded by 6 months of infant stimulation (Group B). The infant-stimulation program included motor, sensory, language, and cognitive activities of increasing complexity. Masked outcome assessment was performed after both 6 and 12 months of therapy to evaluate motor quotient, motor ability, and mental quotient. After six months, the infants in Group A had a lower mean motor quotient than those in Group B (49.1 vs. 58.1, P = 0.02) and were less likely to walk (12 vs. 35 percent, P = 0.07). These differences persisted after 12 months of therapy (47.9 vs. 63.3, P less than 0.01, and 36 vs. 73 percent, P = 0.01, respectively). We noted no significant differences between the groups in the incidence of contractures or the need for bracing or orthopedic surgery. Group A also had a lower mean mental quotient than Group B after six months of therapy (65.6 vs. 75.5, P = 0.05). The routine use of physical therapy in infants with spastic diplegia offered no short-term advantage over infant stimulation. Because of the limited scope of the trial, our conclusions favoring infant stimulation are preliminary. The results suggest that further study of the effects of both physical therapy and infant stimulation is indicated.