The objective of this study was to determine the hematopoietic effects and toxicity of low-dose granulocyte colony-stimulating factor (G-CSF) in myelodysplastic syndrome (MDS) patients with neutropenia. Recombinant human G-CSF (Lenograstim) was administered by daily subcutaneous injection with an initial dosage of 0.5 microg/kg per day for 2 weeks. Patients not responding to the initial dosage received the escalated dosage, 1 to 2 microg/kg per day for 2 weeks. Eligibility criteria were the following: French-American-British disease classification subtype refractory anemia (RA), refractory anemia with ringed sideroblasts (RARS), or refractory anemia with excess blasts (RAEB) with an absolute neutrophil count (ANC) of <1.5 x 10(9)/L. Criteria indicating response to treatment were ANC of >1.5 x 10(9)/L and doubling of ANC on at least 2 occasions. Thirty-two MDS patients were recruited from 6 university hospitals. Eighteen patients had RA, 4 had RARS, and 10 had RAEB. Median age was 56.4 years (range, 28-87 years). Twenty-six patients (81.2%) had an increase in ANC from a median of 0.94+/-0.35 x 10(9)/L to 4.24+/-3.78 x 10(9)/L. Three of 6 patients who did not respond to the initial dosage responded to the escalated dosage of 1 microg/kg per day. Eighteen (81.8%) of 22 patients with RA or RARS responded compared with 8 (80%) of 10 patients with RAEB. The response rates in patients with ANCs of <0.5 x 10(9)/L. 0.5 to <1.0 x 10(9)/L, and 1.0 to 1.5 x 10(9)/L were 80%, 70%, and 88.2%, respectively. The side effects were minimal. No significant changes in hemoglobin levels or platelet counts were observed. In conclusion, low-dose G-CSF administered by subcutaneous injection is well tolerated and effective in improving neutropenia in MDS patients.