Pilot testing is commonly used to evaluate the site-specific effectiveness of ultra violet (UV) disinfection. Most of these tests employ relatively small pilot UV hardware. Although this approach may provide qualitative or semiquantitative performance data, full-scale design frequently relies on empirical data and conservative design decisions. For these reasons, large-scale pilot systems offer advantages relative to small-scale systems. Numerical calculations of UV intensity distributions illustrate a distinct advantage for long lamps due to the end effects. The hydraulic behavior of small-scale systems may be distorted by boundary-layer effects. Geometric and dynamic similarity between large-scale pilot systems and full-scale installations dictates hydrodynamic similarity. Large-scale pilot testing allows more accurate evaluations of UV process performance and less conservative design decisions for scale-up.