Hydroxocobalamin combines with cyanide to form cyanocobalamin. We hypothesised that the amount of cyanocobalamin formed after a fixed dose of hydroxocobalamin given for cyanide poisoning would correlate with initial blood cyanide concentration. We determined blood cyanide concentration in 12 patients exposed to residential fires, and compared this with markers of the amount of cyanocobalamin formed after treatment with 5 g intravenous hydroxocobalamin. All relationships were highly correlated (r2 0.79-0.95), for the whole group, and there appeared to be an almost linear relationship for the 9 patients with initial cyanide concentration below 40 mumol/L. Above this concentration, no further cyanocobalamin was formed from a single 5 g dose of hydroxocobalamin. In one patient with initial blood cyanide concentration of 96 mumol/L, however, plasma cyanocobalamin concentration approximately doubled after a second 5 g dose of hydroxocobalamin. 5 g of hydroxocobalamin appears capable of binding all available cyanide ions for blood cyanide concentrations up to about 40 mumol/L. Beyond this, more hydroxocobalamin must be given for remaining cyanide ions to be bound. This information will allow clinicians to use rapidly measurable plasma cyanocobalamin concentrations to gauge severity of exposure and evaluate adequacy of treatment.