The diagnostic value of determination of the anion and osmolal gaps was studied in 6 patients poisoned with methanol and in 5 poisoned with ethylene glycol. Increased osmolal gap was present on admission in all patients, whereas increased anion gap was present in all except one. In the methanol-poisoned patients the mean blood values were: pH 7.27, anion gap 24 mmol/l, osmolal gap 81 mosmol/kg H2O, methanol 67 mmol/l, ethanol 11 mmol/l and in the ethylene glycol-poisoned patients: pH 6.93, anion gap 38 mmol/l, osmolal gap 35 mosmol/kg H2O and ethylene glycol 24 mmol/l. In the absence of alcoholic acidosis or diabetic coma the finding of a simultaneous increase in both the anion and osmolal gaps indicates methanol or ethylene glycol poisoning. Thus determinations of the anion and osmolal gaps are mandatory whenever facing metabolic acidosis of unknown etiology.