The purpose of this study was to investigate the effect of six months strength training with or without supplementing protein and vitamins, on chromosomal integrity of buccal cells in institutionalized elderly. One hundred seventeen women and men (65–98 years) performed either resistance training (RT), RT combined with a nutritional supplement (RTS) or cognitive training (CT) twice per week for six months. Participants’ fitness was measured using the 6 min walking, the chair rise, and the handgrip strength test. Genotoxicity and cytotoxicity parameters were investigated with the Buccal Micronucleus Cytome (BMcyt) assay. Six minutes walking and chair rise performance improved significantly, however, no changes of the parameters of the BMcyt were detected. Age and micronuclei (MN) frequency correlated significantly, for both women (r = 0.597, p = 0.000) and men (r = 0.508, p = 0.000). Squared regressions revealed a significant increase in the MN frequency of buccal cells with age (R2 = 0.466, p = 0.000). Interestingly and contrary to what was shown in blood lymphocytes, chromosomal damage in buccal cells increases until very old age, which might qualify them as a valid biomarker for aging. Unexpectedly, in this group of institutionalized elderly, resistance training using elastic bands had no effect on chromosomal damage in buccal cells. Display omitted •Mutation in buccal cells increased until very old age – a new aging biomarker?•Chromosomal damage in buccal cells was age-dependent and equal for women and men.•Strength training improved fitness but not mutagenicity in buccal cells of elderly.