The aim of this study was to provide objective data on the cardiopulmonary fitness and physical activity patterns of Northern Irish postprimary schoolchildren. Forty-five children (23 boys, 22 girls), ages 11-16 years, took part in this study. Each child performed a laboratory test of peak aerobic power (PVO2) and had his/her heart rate monitored for up to 4 school days. The mean values of PVO2 in both boys and girls were in keeping with previous literature. No significant difference was observed between boys and girls in terms of total activity (>50% PVO2), but boys engaged in significantly more vigorous activity (>70% PVO2 than girls did (p<0.05). Younger boys engaged in significantly more vigorous activity than both older boys (p<0.01) and younger girls (p<0.05). A significant negative correlation was found between age and total activity for boys (r= −0.476, p<0.05), but not for girls (r= -0.173, n.s.). The surprisingly low levels of physical activity on the part of older children of both sexes are a cause for concern.