Hartnup disease was diagnosed in 12 children and 3 of their 15 sibs in the course of routine urine screening of 6-week-old infants in New South Wales. These children were followed for up to 8 years, during which time there were only two clinical episodes which might be ascribed to Hartnup disease. The mental development of all the children was normal. 10 had height centiles less than the midparent height centiles, while 4 had centiles equal to or above the midparent centiles. The study shows that in children with Hartnup disease in Australia symptoms are very uncommon. Mental development is normal, and heights are possibly slightly below that expected. Hartnup disease has an incidence of approximately 1 in 33 000 in New South Wales.