Myonecrosis has been reported to occur in patients with carbon monoxide (CO) poisoning, and last year we reported a case of non-traumatic rhabdomyolysis in a patient with CO poisoning secondary to smoke inhalation. We prospectively studied the association between CO poisoning and rhabdomyolysis by obtaining serum creatine kinase (CPK) levels on 65 of 81 consecutive patients (range 20-1315 IU/L) who presented to the University of Illinois Hospital Emergency Room during a 3-month period with CO levels greater than 5.0% (range 5.0%-63.9%). Thiocyanate levels were obtained on 45 patients (range 0-3.5 mg/dl). We found no statistically significant correlation by linear regression analysis between CO level and CPK level in these patients. A subjective complaint of weakness was obtained in 4 patients and physical evidence of weakness was found in 1 of these (this was felt to be secondary to a cerebrovascular accident). In none of these 4 patients was an elevated CPK level noted. We did, however, note an association between thiocyanate level and CPK level by linear regression analysis (p less than 0.02). A power curve was a better fit for this data (r2 = 0.7). This data suggests that serum CPK levels should not be routinely obtained on patients with CO poisoning and that cyanide may play a more important role in non-traumatic rhabdomyolysis associated with toxic inhalation than had previously been suspected.