Abstract Purpose: To investigate the feasibility of adaptive dosing and the impact of pharmacogenetic variation on 13-cis-retinoic acid (13-cisRA) disposition in high-risk patients with neuroblastoma. Experimental Design: 13-cisRA (160 mg/m2 or 5.33 mg/kg/d) was administered to 103 patients ages 21 years or less and plasma concentrations of 13-cisRA and 4-oxo-13-cisRA quantitated on day 14 of treatment. Seventy-one patients were recruited to a dose adjustment group, targeting a 13-cisRA Cmax of 2 μmol/L, with dose increases of 25% to 50% implemented for patients with Cmax values less than 2 μmol/L. A population pharmacokinetic model was applied and polymorphisms in relevant cytochrome P450 genes analyzed. Results: 13-cisRA Cmax values ranged from 0.42 to 11.2 μmol/L, with 34 of 103 (33%) patients failing to achieve a Cmax more than 2 μmol/L. Dose increases carried out in 20 patients in the dose adjustment study group led to concentrations more than 2 μmol/L in 18 patients (90%). Eight of 11 (73%) patients less than 12 kg, receiving a dose of 5.33 mg/kg, failed to achieve a Cmax of 2 μmol/L or more. Significantly, lower Cmax values were observed for patients treated with 5.33 mg/kg versus 160 mg/m2 (1.9 ± 1.2 vs. 3.1 ± 2.0 μmol/L; mean ± SD; P = 0.023). Cmax was higher in patients who swallowed 13-cisRA capsules as compared with receiving the drug extracted from capsules (4.0 ± 2.2 vs. 2.6 ± 1.8 μmol/L; P = 0.0012). The target Cmax was achieved by 93% (25/27) versus 55% (42/76) of patients in these 2 groups, respectively. No clear relationships were found between genetic variants and 13-cisRA pharmacokinetic parameters. Conclusions: Dosing regimen and method of administration have a marked influence on 13-cisRA plasma concentrations. Body weight–based dosing should not be implemented for children less than 12 kg and pharmacologic data support higher doses for children unable to swallow 13-cisRA capsules. Clin Cancer Res; 19(2); 469–79. ©2012 AACR.