This paper describes the results of long-term fouling tests for cooling tower water flowing inside enhanced tubes. The data were taken using 800 ppm calcium hardness water supplied to an operating chiller/cooling tower system. The fouling mechanism is a combination of precipitation fouling and particulate fouling. Fouling data were measured for seven different enhanced tube geometries over a 2500 h operating period. The tubes tested had internal helical ridges, and the number of ridge starts varied from 10 to 45. Little fouling occurred until 1300 h. After 1300 h, moderate fouling began to occur in the tubes having a large number of starts. These tubes also provide the highest water-side heat transfer coefficients. Significant fouling existed in the tubes having 30-45 starts at the end of the test period. The salient finding of this work is that the potential for fouling increases as the number of starts and helix angle increases.