The study of physical activity patterns (PAP) is still in its infancy due to methodological problems in measuring instruments of physical activity, and the lack of specific analytical tools to capture all its intrinsic issues. Moreover, it is not well known if differences among subjects are mainly due to specific environmental conditions, genetic attributes or both. This picture is more acute in children. PURPOSE: to explore different ways of viewing and analyzing PAP in children. METHODS: 31 twin pairs (18 MZ and 13 DZ) aged 6 to 12 years of age were monitored for 5 days (3 week-day and a week-end) with a tri-axial accelerometer (TRITRAC R3D). Count data was read in a specialized software (GEMWIZARD), was transformed in energy expenditure according to manufacturer indications, so that 4 phenotypes were derived: till 3 Mets (low PA), from 3 to 6 Mets (moderate PA), from 6 to 9 Mets (vigorous PA), and above 9 Mets (very vigorous PA). SPSS was used for all data analysis (descriptive stats, graphical dispays and correlations). RESULTS: It is evident that most part of the days is spent in low PA, and very few episodes of vigorous or very vigorous activities. Per each hour, across days, about 56 minutes are of low PA. Per day, about 17 minutes are spent in moderate PA, and 3 to 4 minutes in vigorous or very vigorous PA. The amount of inter-individual differences in pairs of MZ and DZ twins is very high, although intra-pair range seems higher in DZ than MZ twins. Twin similarity (Pearson correlation) is as follows: low PA, rMZ=0.886, rDZ=0.772; moderate PA, rMZ=9.946, rDZ=0.880; vigorous PA, rMZ=0.636, rDZ=0.490; very vigorous PA, rMZ=0.759, rDZ=0.522). CONCLUSIONS: In children, physical activity is mainly random. The highest frequency is for low activities (∼3 Mets), interspaced with a very low frequency of moderate to vigorous PA. In twins the intra-pair differences are higher in DZ than MZ twins, suggesting that not only environmental factors are responsible for such RESULTS. Genetic factors may be also important. Adequate intervention programs should focus on inter-individual differences and diverse responses are to be understood in the light of genetic differences. Moreover, it is also important to provide children ample opportunities for doing more frequently moderate and vigorous PA through games and formal Physical Education programs in schools.