The primary aim of the study is to experimentally investigate the stability performance of antifer units on the trunk section of breakwaters under the effect of regular and irregular waves in case of irregular placement. The stability performance tests were conducted for different slopes, i.e. cot alpha = 1.25, 1.5, 2.0, 2.5, under irregular waves and for cot a = 2.5 under regular waves. Hudson's formula was employed in order to characterize the stability performance of antifer units for the irregular placement technique. Different representative wave height parameters, i.e. H-s, H-1/10 and H-max, were examined to determine the one best characterizing breakwater stability. Furthermore, the effects of wave period and wave steepness on the stability of the breakwater were explored. (C) 2003 Elsevier Ltd. All rights reserved.