To solve the equation \(\sqrt{-4x} = 100\), we can start by squaring both sides to eliminate the square root:
\[ -4x = 100^2 \]
Calculating \(100^2\):
\[ -4x = 10000 \]
Next, we can solve for \(x\) by dividing both sides by \(-4\):
\[ x = \frac{10000}{-4} = -2500 \]
Thus, the solution is:
\[ \boxed{-2500} \]