The growth of eosinophil progenitors (CFU‐Eo) and its modulation by hydrocortisone (HC), mononuclear cells, interleukin 1 (IL‐1), and interleukin 2 (IL‐2) in three cases of eosinophilia are reported in this paper. One m̈M HC decreased the proportion of CFU‐Eo in each of these three patients, and in each case, the addition of autologous mononuclear cells at a 1:1 ratio abrogated the effect of HC on CFU‐Eo. As studied in two of the three cases, IL‐1 and IL‐2 were able to prevent the effect of HC. Further studies showed no effect of HC when monocyte T cell‐depleted marrow cells were used as the target population. These results suggest that CFU‐Eo production in eosinophilic states is subject to modulation by HC and T lymphocytes.