What predisposes some cocaine users to its complications is not known. Because cocaine is metabolized by plasma cholinesterase enzyme, we evaluated this enzyme in 14 patients with cocaine-induced complications, 11 long-term cocaine users from Bolivia, 14 persons in the United States without any documented cocaine-induced complications, and 14 subjects who have not used cocaine. The enzyme was found to be significantly lower in patients with cocaine-induced complications as compared to other groups (p < 0.05). A low level of plasma cholinesterase enzyme may predict complications from cocaine use.