Children under 5 years of age residing in an urban settlement of Papua New Guinea were monitored from May 1987 to July 1988 in an attempt to identify aetiological factors of childhood diarrhoea. Low birthweight was found to be strongly associated with diarrhoea (incidence density ratio (IDR) = 1.60, 95% confidence interval (CI) = 1.26-2.03). The low birthweight effect was noticeable to at least 3 years of age. Greater attention should be paid to reducing the incidence of low birthweight, because such a reduction will not only be of benefit in the control of diarrhoea, but will also alleviate other factors which contribute to infant morbidity and mortality.