Objective: To investigate growth of children with attention-deficit/hyperactivity disorder (ADHD) in the Preschool ADHD Treatment Study (PATS) before and after initiation of treatment with methylphenidate at titrated doses (average, 14.2 mg/day) administered three times daily, 7 days/week for approximately equal to 1 year. Method: The heights and weights of 140 children with ADHD were measured up to 29 times in the PATS protocol, starting at an average age of 4.4 years. The relationship between standard (z) scores and time on medication was examined using mixed-effect regression to estimate change in relative size (slope). Results: Average relative size at baseline was significantly (p less than 0.0001) greater than zero for z height (+0.45) and z weight (+0.78), indicating greater than expected height (by 2.04 cm) and weight (by 1.78 kg). During treatment, slopes were significantly (p less than 0.0001) less than zero for z height (-0.304/yr) and z weight (-0.530/yr), indicating reduction of growth rates. For 95 children who remained on medication, annual growth rates were 20.3% less than expected for height (5.41 cm/yr - 6.79 cm/yr = -1.38 cm/yr) and 55.2% for weight (1.07 kg/yr - 2.39 kg/yr = -1.32 kg/yr). Conclusions: Risks of reduced growth rates should be balanced against expected benefits when preschool-age children are treated with stimulant medication. (Contains 2 figures and 2 tables.)