Objective: This study examined the association between 5-aminolevulinic acid (5-ALA) and glucose tolerance. Design: A double blinded, randomized prospective parallel-group comparison study. Setting: Participants were recruited from the community in Honolulu, Hawaii, using radio and TV ads, and at community events. Participants: One hundred fi fty-four males and females ages 40-70 years, with evidence of prediabetes: hemoglobin A1c (HbA1c) 5.8%-7.0% at the screening visit. Intervention: Participants were randomized equally to one of three study groups: (1) low dose 5-ALA supplement (15 mg capsule); (2) high dose 5-ALA (50 mg capsule); and (3) control (placebo capsule of identical size and color). Main outcome measures: HbA1c and 2 hours post-oral glucose tolerance test (OGTT) glucose levels. Results: Among individuals taking 5-ALA supplements for 12 weeks, 2 hours post-OGTT glucose levels declined signifi cantly compared to those not taking the supplement ( p = 0.02). The relationships were stronger among those with baseline glucose intolerance, or 2 hours post-OGTT glucose measurements greater than 140 mg/dL ( p = 0.005 and p = 0.02 for the low and high dose group, respectively). Similar trends were observed for HbA1c but results were of borderline signifi cance ( p = 0.07). No untoward effects were reported. Conclusions: Further studies are indicated. The potential benefi ts of 5-ALA dietary supplementation are affi rmed by this investigation.