Trough serum methadone concentration was measured in 43 patients under treatment for heroin addiction and complaining of withdrawal symptoms. Low serum levels were noted only in patients taking very low doses and in 10 patients who were concomitantly using enzyme-inducing drugs. The 27 patients in the maintenance program who had trough levels greater than 100 ng/ml were given no dose increase and were followed up prospectively. Alternate explanations for the patient's symptoms were well accepted in almost all cases, and subsequent performance in the treatment program appeared to be independent of serum level. We conclude that a trough serum level of 100 ng/ml is adequate for effective methadone maintenance. Measurement of serum levels can be a valuable intervention in patients with difficulties.