Clinical effects of purified human urinary colony-stimulating factor (CSFHU) were studied in nine patients with chronic neutropenia of childhood who ranged in age from 1 to 18 years. The patients were given CSFHU at a dose of 6 x 10(6) U/m2/d for seven consecutive days, and their peripheral neutrophil counts were serially followed for 6 to 12 months. In addition, changes in bone marrow morphology and granulocyte- macrophage colony-forming cells (GM-CFUs) were studied. Transient increases in neutrophil counts occurred during the first 4 weeks in all patients except one. Repeated increases were seen in a cyclic fashion in five patients. Three of the patients recovered from the neutropenia after several cycles of fluctuation of neutrophil counts; the counts (per microliter) reached 5,880, 5,640, and 2,000 as compared with pretreatment levels (mean +/- 2 SD) of 80 +/- 164, 182 +/- 250, and 2 +/- 14, respectively. The percentage (mean +/- SE) of marrow neutrophilic cells increased from 30.4% +/- 7.0% to 34.9% +/- 6.9% (P less than .05) and the ratio of the later neutrophil precursors to the earlier ones from 1.02 +/- 0.27 to 1.42 +/- 0.36 (P less than .02) 2 weeks after CSFHU infusion. There were no changes in GM-CFU numbers. These results indicate that CSFHU can increase neutrophil counts by increasing the number and maturity of the marrow neutrophil precursors in some types of childhood chronic neutropenia.