The development of secondary enuresis was examined in a birth cohort of New Zealand children studied to the age of 10 years. By this age, 7.9% of children had developed secondary enuresis. Analysis suggested two risk factors were involved in secondary enuresis. First, children who were late to attain nocturnal bladder control had significantly higher risks of secondary enuresis than children who attained early nocturnal bladder control. Secondly, the child's level of exposure to adverse life events was associated with the onset of secondary enuresis. Children who attained bladder control after 5 years were 3.39 (1.76-6.56) times more likely to develop secondary enuresis than those who attained bladder control before the age of 3 years (P less than 0.001); children who were exposed to four or more life events in a given year were 2.56 (1.18-5.50) times more likely to develop secondary enuresis in that year (P less than 0.05). These results suggest that the rate at which the child acquires primary bladder control acts as a vulnerability factor which determines the child's susceptibility to developing secondary enuresis when exposed to stress.