In this cross-sectional prevalence study in 1503 14-year-old Finnish schoolchildren (n = 1503) low back pain was found to be the third most common form of pain interfering with schoolwork or leisure time during the past 12 months. The lifetime cumulative incidence of low back pain was 30% and that of sciatica 1.8%. Of the 417 pupils who had experienced low back pain at some time, 39% had suffered during the past month; 65% recovered in one month from the last pain episode, while 35.2% of those reporting disabling low back pain during the past year were aware of recurrent or continual pain. Thus, 7.8% (n = 107) of the whole population could be classified as "low back pain chronics": 86% of the low back pain chronics had trouble with at least one of the daily activities listed, most commonly with sitting at school. Excluding pain in the extremities or sciatica, girls reported various pains more commonly than boys. Moreover, girls reporting recurrent low back pain had more trouble with their daily activities due to pain than boys (p < 0.001), even though the recovery from the last pain episode took the same time in boys and girls. The pupils who had had sciatica at some time, in addition to recurrent low back pain, had more trouble with the 10 daily activities than others with recurrent low back pain (p = 0.014).