Immunological functions of the tonsils and possible effects of their removal are still controversial. One reason for this is the lack of long-term follow-up investigations after tonsillectomy. In the present study selected parameters of the cellular and humoral immune systems of 160 children 0.5-11 years after tonsillectomy (mean 6.6 +/- 2.1 years) were compared to those of 302 age-matched non-tonsillectomized children. In tonsillectomized children the incidence of infections of the upper respiratory tract was not increased compared to the non-tonsillectomized control group. Slightly increased percentages of CD 21 + cells, raised counts of CD4+ cells, absolute and relative increases in DR+ cells and a raised CD4+ DR count was found mainly in tonsillectomized boys, while lymphocyte subpopulations of tonsillectomized girls remained unaffected. Tonsillectomized children had lower IgA levels, but the complement system was not altered in either sex. These findings show that while tonsillectomy may lead to certain changes in the cellular and humoral immune systems, these alterations are clinically insignificant and no increased frequency of immunomodulated diseases should be expected.