The indirect hemagglutination (IHA) and immunoelectrophoresis (IEP) tests were used for diagnosis and follow-up evaluation of 17 patients with alveolar hydatid disease caused by Echinococcus multilocularis. Follow-up periods ranged from 2 to 22 years. At the time of diagnosis 16 (94%) patients' sera gave IHA titers ≥ 1:128. Serum specimens from 13 patients were examined by IEP; nine (69%) revealed the arc 5, and three of the four arc 5-negative sera revealed one or more non-characterized bands. Titers declined markedly during the first year following radical surgical resection of the larval lesions; in three cases clinical evidence of recurrence was preceded by rising serologic titers. Antibody has persisted at high levels in nonresected patients treated continuously with high doses of mebendazole.