In a right-angled triangle in which all side lengths are integers, one has a cathetus length $1994$. Determine the length of the hypotenuse.
1. Given a right-angled triangle with integer side lengths, one of the legs (catheti) is \(1994\). We need to determine the length of the hypotenuse.
2. Let the other leg be \(a\) and the hypotenuse be \(b\). According to the Pythagorean theorem, we have:
   \[
   1994^2 + a^2 = b^2
   \]
3. Rearrange the equation to isolate \(1994^2\):
   \[
   1994^2 = b^2 - a^2
   \]
4. Notice that \(b^2 - a^2\) can be factored using the difference of squares:
   \[
   1994^2 = (b - a)(b + a)
   \]
5. Let \(x = b - a\) and \(y = b + a\). Then:
   \[
   1994^2 = xy
   \]
6. Since \(1994 = 2 \times 997\), we have:
   \[
   1994^2 = (2 \times 997)^2 = 4 \times 997^2
   \]
   Therefore:
   \[
   xy = 4 \times 997^2
   \]
7. To find integer solutions for \(x\) and \(y\), we consider the factor pairs of \(4 \times 997^2\). The simplest pair is \(x = 2\) and \(y = 2 \times 997^2\):
   \[
   x = 2, \quad y = 2 \times 997^2
   \]
8. Solving for \(b\) and \(a\) using \(x\) and \(y\):
   \[
   b - a = 2 \quad \text{and} \quad b + a = 2 \times 997^2
   \]
9. Adding these two equations:
   \[
   (b - a) + (b + a) = 2 + 2 \times 997^2
   \]
   \[
   2b = 2 + 2 \times 997^2
   \]
   \[
   b = 1 + 997^2
   \]
10. Subtracting the first equation from the second:
    \[
    (b + a) - (b - a) = 2 \times 997^2 - 2
    \]
    \[
    2a = 2 \times 997^2 - 2
    \]
    \[
    a = 997^2 - 1
    \]
11. Therefore, the hypotenuse \(b\) is:
    \[
    b = 997^2 + 1
    \]

The final answer is \(\boxed{994010}\).