Growth of low density human cord blood cells in long‐term suspension culture was evaluated at incubation conditions of 5% carbon dioxide in ∼ 20% (normal incubator) oxygen tension or in 5% (low) oxygen tension. During the first 5 weeks there was no difference in the numbers of morphologically recognizable cells grown at either oxygen tension, while growth was superior from weeks 5 to 8 at ∼ 20% oxygen tension. For the first 5 weeks, growth of granulocyte‐macrophage progenitors (CFU‐GM) was superior at 5% oxygen tension during the long‐term and semi‐solid culture phases and least impressive in the long‐term and semi‐solid cultures incubated at ∼ 20% oxygen. This trend was reversed after 5 weeks and after 8 weeks there were no detectable CFU‐GM in suspension cultures at 5% oxygen while steady state levels of CFU‐GM were maintained for greater than 12 weeks in suspension cultures at ∼20% oxygen. Semi‐solid cultures for erythroid (BFU‐E) and multipotential (CFU‐GEMM) progenitors were incubated at ∼ 20% oxygen only. During the first 4 weeks, the growth of BFU‐E and CFU‐GEMM was superior at 5% oxygen during long‐term culture. Numbers of these cells decreased by week 5 and at this time growth was better in the long‐term cultures grown at ∼ 20% oxygen. By week 6, no BFU‐E or CFU‐GEMM were detectable. Thus, growth of cord blood at low oxygen tension gives an initial enhancement in output of progenitor cells, but this appears to be at the expense of continued production.