Antifreeze ingestions require rapid and accurate differential diagnosis to prevent fatal outcomes. Sodium fluorescein is added to some commercial antifreeze preparations (ethylene glycol) to a final concentration of approximately 20 micrograms/mL as a colorant to aid in detection of automobile cooling-system leaks. For an adult human being, a potentially toxic volume of antifreeze is 30 mL, which contains 0.4 to 0.6 mg sodium fluorescein. Six male volunteers were given a 0.6-mg oral bolus of sodium fluorescein on an empty stomach. Urine was collected at two-hour intervals. Using a Wood's lamp, visually detectable fluorescence was seen with 100% reliability for two hours and 60% reliability for four hours. A second group of male volunteers was given the same dose of sodium fluorescein, and fluorescence was measured with a fluorometer during a six-hour period. Detectable fluorescence was present in all samples except the zero time point, including those with no fluorescence present by visual examination. We conclude that exposing urine to a Wood's lamp may be a useful adjunctive diagnostic test for early evaluation of patients with suspected antifreeze ingestion while awaiting definitive quantitative analysis of serum ethylene glycol concentration. A prospective clinical trial is needed to evaluate the frequency of false-positives and false-negatives.