Three thousand, eight hundred and eighty-two (3,882) children in grades 2-5, attending 16 rural primary and all-age schools in central Jamaica were weighed and their weight-for-age standard deviation scores calculated using the World Health Organization/National Center for Health Statistics (WHO/NCHS) references. Heights were also measured in a random sample of the grade 5 children (n = 793) and height-for-age and body mass index (BMI-kg/m2) calculated. Sixty-nine per cent of the total sample were of normal weight-for-age, 2% were moderately undernourished (weight-for-age > -3 Z-score, < or = -2 Z-score), and a further 24% mildly undernourished (weight-for-age > -2 Z-score, < or = -1 Z-score). Few children were overweight. The frequency distribution of weight-for-age was similar in girls and boys. In the subsample of children in whom heights were measured, 25.8% were < or = -1 Z-score height-for-age, and of these 4.9% were < -2 Z-score. Compared with a survey conducted in a similar rural area in the 1960s, the children's mean weights for age group categories were 1.1 to 3.7 kg heavier. Children who were older than appropriate for their grade were more likely to be undernourished (Odds ratio 3.94, 95% CI 3.21, 4.83), which suggests that undernourished children may be more likely to repeat a grade or start school later.