Objective To investigate the correlation between colony-stimulating activities of serum from patients with severe aplastic anemia (SAA) and their responses to immunosuppressive therapy (IST).Methods The effects in vitro of a total 50 test serum samples from SAA patients before and after IST, and from normal subjects on healthy human marrow colony growth of BFU-E and CFU-GM were examined to reflect their burst promoting activities (BPAs) and granulocyte/macrophage colony-stimulating activities (GM-CSAs). Serum erythropoietin (Epo) level was also measured with ELISA method before and after IST in SAA patients.Results The results showed that the BPAs of sera from SAA patients before IST were higher significantly than that of normal controls (P<0.001), after IST, the BPAs of sera from SAA patients had no obvious changes. Serum GM-CSAs from 13 of 22 SAA patients were normal, and the other 9 patients were extremely lower compared with normal subjects; after IST, their serum GM-CSAs also had no obvious changes. Serum Epo concentrations in SAA patients both at diagnosis and after IST were higher significantly than normal (P<0.001); however, serum concentrations declined in responded patients (P>0.05), while further increased in nonresponded patients (P>0.05).Conclusion Serum GM-CSA of SAA patients was a predictive factor for responsiveness to IST, and a normal value was associated with a good response.