An ultrasound study in elderly patients (> or =65 years) showed that body mass index (BMI) was strongly correlated with deltoid subcutaneous layer thickness in males (r = 0.69 dominant arm, 0.71 non-dominant arm) and females (r = 0.79 both arms). Females with the same BMI as males had significantly thicker subcutaneous layers (p = 0.0001) and thinner muscle layers (p = 0.0003). Minimal needle length required for deltoid intramuscular injection where the needle was entered at 90 degrees to the long axis of the humerus was defined by BMI group. In all BMI males and females, BMI <35, intramuscular injection could be achieved with a 25 mm long needle, whilst in females BMI >35, a 32 mm long needle is required. These data will be used in studies to resolve the clinical equipoise regarding the optimal route of administration (intramuscular versus subcutaneous) of vaccines (e.g. influenza and pneumococcal vaccines), which are provided through public health programs for the elderly.