This study examined the relative contribution of contingent payment and worksite CO monitoring to the long-term maintenance of smoking abstinence. Forty-seven hospital employees who had abstained from smoking for five days (confirmed by CO analysis) were randomly assigned to one of three follow-up groups: (a) contingent payment/frequent monitoring (n = 17); (b) noncontingent payment/frequent monitoring (n = 16); or (c) non-contingent payment/infrequent monitoring (n = 14). Contingent payment combined with frequent CO monitoring delayed but did not ultimately prevent subjects relapse to smoking by the end of the six month follow-up. Contingent subjects maintained CO values less than or equal to 11 ppm significantly longer than did either the Non-contingent or the Control subjects (p = .03). CO monitoring alone had no effect on abstinence outcomes; both Noncontingent and Control subjects showed high rates of early relapse.