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We previously reported strain-specific susceptibility to dexamethasone-induced osteonecrosis in mice. Here we report that BALB/cJ and BALB/cAnNHsd mice display substrain-specific differences in dexamethasone-induced adverse effects. As compared with BALB/cJ mice, BALB/cAnNHsd weighed more (16.6 g compared with 13.7 g) at the beginning of dexamethasone administration on postnatal day 28 and fewer died during the dexamethasone regimen (10% compared with 50%). Although the 2 substrains had similar plasma concentrations of dexamethasone, BALB/cJ mice were more susceptible to developing dexamethasone-induced osteonecrosis. A higher dose of dexamethasone (8 mg/L) throughout the treatment period compared with a lower dose (8 mg/L loading dose during week 1 followed by 4 mg/L for the remainder of the treatment period) and earlier start of treatment (postnatal day 24 compared with postnatal day 28) was required to induce osteonecrosis with a similar frequency in BALB/cAnNHsd mice as in BALB/cJ mice. Our results show, for the first time, substrain-specific differences in the development of osteonecrosis in mice.