The Dutch consortium "NoordzeeWind" has built the first offshore wind farm in Dutch North Sea waters, known as "Offshore Wind farm Egmond aan Zee" (OWEZ) Part of the construction works consisted of driving 36 monopiles into the sea floor, during April-June 2006. The noise levels that attended this activity may have been detrimental to marine life forms. Cetaceans in particular are sensitive to very high noise levels and a possible impact on the most abundant cetacean living off the Dutch coast, the harbour porpoise was therefore studied. Direct observations were hard to conduct, given the low (summer) densities of porpoises around the construction site. Pathological observations on stranded specimen failed to produce clear results. Inner ears of freshly stranded porpoises were examined for possible damage, but before necropsies could be conducted the animals had been stored frozen and this had destroyed any visible signs of noise-induced damage to the inner ear. Thus, the spatio-temporal pattern of porpoise strandings was examined. Porpoises did not strand in higher numbers on the coastal stretch directly east of the construction site, or to the north-east of this lo cation (downstream) compared to other parts of the country. Porpoises also did not strand in higher than expected numbers near the construction site, at the time of construction. It was therefore concluded that the construction did not lead to visibly increased mortality of harbour porpoises. In retrospect this might have been expected, given that densities of porpoises are normally very low in summer at the site, that the building process is noisy anyway, scaring porpoises off (to safe distances) before the actual pile driving commences. A ramp-up procedure and usage of a pinger further helped to ward off porpoises from the site, before full-power pile driving started. These factors combined (timing and high before-pile driving noise levels) made it very unlikely that porpoises got in harm’s way during the construction of OWEZ.