In the course of clinical trials of tonsillectomy, adenoidectomy, and adenotonsillectomy, we measured serum levels of immunoglobulins G, A, and M by rate nephelometry in 268 subjects at baseline and at intervals after receipt or nonreceipt of surgery. Up to 30 months after baseline we observed a consistent pattern of change only in levels of immunoglobulin G, decreases in which tended to occur more commonly and be of greater magnitude in subjects who received tonsillectomy or adenotonsillectomy than in corresponding control subjects. However, in both surgical and control subjects the subsequent incidence of throat infection showed no relationship to either contemporaneous immunoglobulin levels or changes from baseline levels. A decline in serum immunoglobulins following tonsillectomy appears not to be a risk factor for developing throat infection, and may reflect a reduction in antigenic stimulation.