Immunologic studies of 14 girls with Turner syndrome were done before and during treatment with biosynthetic growth hormone (GH). Compared with control subjects, the patients before treatment had a decreased CD4/CD8 ratio and an increased number of cells bearing the natural killer cell marker CD16; serum immunoglobulin levels were within the normal range. During GH treatment some of the girls had a slight reduction in the percentage of CD20+ B cells, but we observed no impairment of B lymphocyte function as demonstrated by the normal in vivo antibody response to the primary antigen Helix Pomatia hemocyanin, administered 6 months after the start of GH treatment. The number of CD16+ natural killer cells returned to normal. Although the number of children with thyroid antibodies increased from two before treatment to five after 1 year, no conclusion about an adverse effect of GH is warranted, because the phenomenon might be part of the natural course of the disease. We conclude that girls with Turner syndrome have minor changes in some immunologic measurements and that GH treatment resulted in some alterations that have no effect on immune function.