We characterize individual and ensemble properties of X-ray flares from stars in the Cygnus OB2 and ONC star-forming regions. We analyzed X-ray light-curves of 1003 Cygnus OB2 sources observed with Chandra for 100ks and of 1616 ONC sources detected in the "Chandra Orion Ultra-deep Project" 850ks observation. We employed a binning-free maximum likelihood method to segment the light-curves into intervals of constants signal and identified flares on the basis of both the amplitude and the time-derivative of the source luminosity. We then derived and compared the flare frequency and energy distribution of Cygnus OB2 and ONC sources. The effect of the length of the observation on these results was investigated by repeating the statistical analysis on five 100ks-long segments extracted from the ONC data.