Ritodrine hydrochloride has been widely used for tocolysis, although serious side-effects have been reported. We report two cases of agranulocytosis induced by ritodrine hydrochloride, which probably occurred by different mechanisms. Two patients were hospitalized because of preterm labor and were given intravenous ritodrine hydrochloride. The nadir of neutrocytes was 199/mm(3) and 13/mm(3) in the two cases, respectively. The total dose of ritodrine hydrochloride was calculated to be 7800 mg for 26 days and 2500 mg for 22 days, respectively. The total doses were heavier and administration duration was longer in Case 1, which suggested a toxic mechanism of agranulocytosis, while in Case 2, they were smaller and shorter, suggesting an immunological mechanism. For patients receiving ritodrine hydrochloride, the white blood cell count should be checked frequently regardless of the duration of therapy and a drug lymphocyte stimulation test should be performed.