OBJECTIVE: The purpose of this study is to investigate the effect of ritodrine hydrochloride on neutrophils, lymphocytes and monocytes in pregnant women with premature labor. STUDY DESIGN: Before completing 37 week gestation, 37 pregnant women with premature labor have been followed up prospectively. Intravenous tocolysis with ritodrine in a dose of 0.2 mg/minute was started immediately and carried on for an average of 4.32 days. While 26 of those were given betamethasone (24 mg/day) therapy (Group I), 11 patients did not get this treatment (Group II). Before and after tocolysis, we obtained the number of total leukocytes, neutrophils, lymphocytes and monocytes. RESULTS: We have found that the total count of leukocytes (r=0.725 p<0.0001) and neutrophils (r=0.46 p=0.011) were reduced, but lymphocytes (r=0.52 p=0.003) were increased significantly without being affected by betamethasone therapy in patients on ritodrine treatment. There was no significant change in the count of monocytes (p>0.05). CONCLUSION: Ritodrine has an effect on the immune system by inducing neutropenia. Especially if we consider the fact that maternal infections are the most important etiologic factors for premature labor, the importance of our result can be seen.