Despite the longstanding clinical use of azathioprine as an immunosuppressive agent in multiple sclerosis, little is known about the action of this drug on a number of parameters of putative pathogenic relevance in the disease. Eleven patients with multiple sclerosis, treated with azathioprine 2.5-3 mg/kg per day, and six untreated patients were studied with serial blood sampling for 1 year. The following immunological parameters were investigated: peripheral blood lymphocyte subsets, natural killer activity, serum IgG, IgM, ICAM-1 and tumour necrosis factor alpha (TNF-alpha). The most relevant changes included a decrease in CD3- CD56+ cells, an increase in CD4+ CD45RA+ cells and a decrease in TNF-alpha levels only in treated patients, while no changes occurred in untreated patients over a 1-year period. The decrease in TNF-alpha levels and the increase in "suppressor-inducer" lymphocytes could reduce chronic inflammation in multiple sclerosis, and paralleled an overall favourable clinical response to azathioprine treatment in our patients.