Objectives: To identify asymptomatic boys with X-linked adrenoleukodystrophy who have a normal magnetic resonance image (MRI), and to assess the effect of 4:1 glyceryl trioleate-glyceryl trierucate (Lorenzo's oil) on disease progression. Method: Eighty-nine boys (mean±SD baseline age, 4.7±4.1 years; range, 0.2-15 years) were identified by a plasma very long-chain fatty acids assay used to screen at-risk boys. All were treated with Lorenzo's oil and moderate fat restriction. Plasma fatty acids and clinical status were followed for 6.9±2.7 years. Changes in plasma hexacosanoic acid levels were assessed by measuring the length-adjusted area under the curve, and a proportional hazards model was used to evaluate association with the development of abnormal MRI results and neurological abnormalities. Results: Of the 89 boys, 24% developed MRI abnormalities and 11% developed both neurological and MRI abnormalities. Abnormalities occurred only in the 64 patients who were aged 7 years or younger at the time therapy was started. There was significant association between the development of MRI abnormalities and a plasma hexacosanoic acid increase. (For a 0.1-μg/mL increase in the length-adjusted area under the curve for the hexacosanoic acid level, the hazard ratio for incident MRI abnormalities in the whole group was 1.36; P=.01; 95% confidence interval, 1.07-1.72.) Results for patients aged 7 years or younger were similar (P=.04). Conclusions: In this single-arm study, hexacosanoic acid reduction by Lorenzo's oil was associated with reduced risk of developing MRI abnormalities. We recommend Lorenzo's oil therapy in asymptomatic boys with X-linked adrenoleukodystophy who have normal brain MRI results.