The incidence rate of giardiasis in New Zealand is one of the highest among developed countries, peaking in the 1–4 year age group. A case–control study was undertaken to identify risk factors for giardiasis among Auckland children under 5 years of age. The exposure history of 69 cases and 98 controls were analysed. Ninety-five per cent cases and 86% controls used water from the Auckland Metropolitan mains (AMM) supply for domestic purpose, 44 cases and 42 controls swam and 59 cases and 54 controls wore nappies. Children wearing nappies were at significantly increased risk of the disease (OR=3·0, 95% CI=1·01–8·9), as were those from households which had more than one child wearing a nappy (6·5, 1·8–23·4). The Auckland metropolitan mains water supply was associated with a reduced risk compared to other drinking water sources. Significantly increased risks were also associated with drinking water consumed away from home (4·7, 2·2–10·1), swimming at least once a week (2·4, 1·1–5·3) and travelling domestically (2·5, 1·03–6·0). The study identified vulnerable groups and modifiable risk factors for diarrhoeal diseases, particularly Giardia infection. Nappy wearing was an independent risk factor for infection. Further study is advocated to ensure better protection of public health, especially for children.