Calculate the percentage decrease in blood glucose from 180 mg/dL to 135 mg/dL.

The original blood glucose level is $180 \text{ mg/dL}$ and the new level is $135 \text{ mg/dL}$.

$$\text{Decrease} = \text{Original Value} - \text{New Value}$$ $$\text{Decrease} = 180 \text{ mg/dL} - 135 \text{ mg/dL}$$ $$\text{Decrease} = 45 \text{ mg/dL}$$

The formula for percentage decrease is:

$$\text{Percentage Decrease} = \frac{\text{Decrease}}{\text{Original Value}} \times 100\%$$ $$\text{Percentage Decrease} = \frac{45 \text{ mg/dL}}{180 \text{ mg/dL}} \times 100\%$$ $$\text{Percentage Decrease} = 0.25 \times 100\%$$ $$\text{Percentage Decrease} = 25\%$$

The percentage decrease is $\boxed{\text{25\%}}$.