Serologic results obtained in 92 cases of toxoplasmic lymphadenopathy diagnosed by lymph node biopsy were reviewed, and guidelines for serologic diagnosis of this disease were established. When tests were first performed within six months of onset of lymphadenopathy, single high titers of IgG toxoplasma antibodies (suggestive of acute infection) were found with the Sabin-Feldman dye test and the direct agglutination test in 93% and 76% of cases, respectively. Observations of significant rises in titer were uncommon because of the late acquisition of sera. Within the first six months after the onset of lymphadenopathy, IgM toxoplasma antibody was demonstrable by the double-sandwich IgM enzyme-linked immunosorbent assay in 88% of cases and by the IgM-immunofluorescent antibody test in 78%. Twenty percent of patients who had serum samples drawn more than 12 months after onset of lymphadenopathy still had IgM toxoplasma antibodies demonstrable by the enzyme-linked immunosorbent assay. No patient first tested six or more months after onset of lymphadenopathy was positive in the IgM-immunofluorescent antibody test. These results provide the basis for recommendations on the use of serologic tests for the diagnosis of acute toxoplasmic lymphadenopathy.