The influence of thymectomy and long term immunosuppression on the phenotype of CD4 T lymphocyte subsets, which were defined by the restricted expression of CD45RA and CD45RO markers, was studied by double immunofluorescence in 29 patients in different clinical stages of generalised myasthenia gravis. In the acute stage of myasthenia, before thymectomy and immunosuppression, no differences in CD4 subsets were observed in the peripheral blood from nine patients and 21 matched controls. Four to seven weeks after thymectomy, there was a slightly decreased proportion of CD4+CD45RO+ (UCHL1+) memory cells (p < 0.05, paired t test). Patients on steroids showed a more pronounced decrease of CD4+CD45RO+ cells suggesting, in addition, a drug-related effect. CD4 subsets (CD45RA, CD45RO, and CD29 positive) in the peripheral blood compartment remained largely stable over 18 to 24 months thereafter. In addition, CD4 subsets were examined in 20 patients with myasthenia gravis who had had a thymectomy between two and 17 years before. With the exception of patients on steroids, there were no differences in CD4 subsets in patients on or off azathioprine. These data did not show any relation of CD4 T cell subsets to the clinical course of myasthenia, or significant changes due to thymectomy, or immunosuppression with azathioprine. These results also complement the authors' clinical experience that thymectomy in adults does not leave a deficit in cell-mediated immunity. The slight change associated with steroid treatment might deserve further attention.