472 Mycophenolate mofetil (MMF) is an immunosuppressive agent that has shown promise in adult patients who have undergone heart transplantation. There have been a number of studies of the pharmacokinetics of MMF in adult solid organ transplant recipients, but there is very little information in the pediatric population. Purpose: To review our experience with MMF dosing and the role of MPA levels for therapeutic drug monitoring in a population of pediatric heart transplant recipients. Data was obtained by review of the pediatric heart transplant database between Nov. 1, 1997 and Oct. 15, 1998. The data included all serum trough MPA levels, patient age, weight, height, indication for and dose of MMF, other medications, and details of all episodes of graft rejection. Forty-four patients (27 males) had a total of 127 serum trough MPA levels. Median age at transplant was 2.7 yrs (0.02 to 18.4 yrs), and at time of review was 6.3 yrs (0.08 to 23.5 yrs). MMF treatment was used for induction in 18 patients, induction and rejection in 23 patients, and graft vasculopathy in 3 patients. Results: There was marked intraindividual (coefficient of variation 6.3-65%) and interindividual variation in MPA level with dosing by weight (R=0.19) and by body surface area (R=0.07), making it difficult to predict the level for any given dose of MMF. There was a significant trend to requiring higher doses to acheive therapeutic drug levels (3-7 ng/mL) in patients less than 5 years of age (125±29 mg/kg for age 0-1 yrs, 101±51 mg/kg for age 1-5 yrs, 49±17 mg/kg for >16 yrs; ANOVA p<0.01). The average dose to acheive therapeutic levels was higher in the immediate post-transplant period (4-8 wks post-transplant, 109±29 mg/kg) than at any time >8 weeks post-transplant (59±34 mg/kg; p<0.001). There were no therapeutic levels at <1 week post-transplant at a similar dose that resulted in therapeutic levels at > 8 weeks post-transplant. There was a trend that MPA levels for a given dose were higher in patients on concurrent FK506 therapy. Conclusions: 1) There is marked intra- and interindividual variation in pharmacokinetics of MMF in pediatric patients, 2) higher MMF doses are required at younger ages and in the early period post-transplant, and 3) lower MMF doses may be required with concurrent FK506 therapy. Therefore, therapeutic drug monitoring of serum trough MPA levels is required for individualization of MMF dosing in pediatrics.