Whole blood viscosity (WBV) and haematocrit (Hct) were measured in 8 very low birth weight premature neonates developing acute severe retinopathy of prematurity (ROP) and 7 control neonates without retinopathy or mild ROP. Single samples were collected at 32-34 weeks conceptual age. There was no statistically significant difference in WBV or Hct between the two groups. The results of this preliminary report suggest that WBV does not appear to play a major role in the development of advanced ROP, though larger studies are required to investigate this further.