The mechanisms of action of most treatments in the autoimmune disorders are unclear and steroids remain the first-line therapy in these diseases. Natural killer T (NKT) cell activity has been implicated in the autoimmune process but whether steroids act via an affect on NKT cell function, such as antigen-specific proliferative capacity, is unknown. Immune thrombocytopenia (ITP) patients were studied ex vivo for NKT cell expansion in response to the specific NKT cell antigen, α-galactosylceramide, before, during or after prednisolone treatment. Prednisolone inhibited antigen-specific NKT cell expansion in ITP patients in remission. Untreated ITP patients also showed reduced NKT cell proliferative capacity, although this was less marked than in treated patients. These results support a role for NKT cells in ITP and aid understanding of the immunosuppressive activities of prednisolone in autoimmune disease.