There is a need to identify early disease markers to facilitate diagnosis of mucopolysaccharidosis type II (MPS II; Hunter syndrome). Mean birth weight and its association with disease severity was investigated in 609 patients enrolled in the Hunter Outcome Survey (HOS). This analysis indicated that birth weight is not an early marker of MPS II and is not associated with disease severity. It remains important to investigate the utility of other factors for early/pre-symptomatic diagnosis.