This study examines the effects of a music therapy (MT) intervention adopting familiar group singing during sundowning, a period of disorientation and/or agitation in the evening hours (Cohen- Mansfield, Garfinkel, and Lipson, 2000), to address the negative mood and non-social behaviour observed in residents diagnosed with dementia. A music therapist, over four consecutive days in the late afternoon, engaged four female residents with dementia, aged 80 to 97, in a small group singing MT session. Two observers, using a mood-behavioural checklist, assessed the residents, (a) during a 15 min pre-test session, (b) a 30 min MT session, and (c) a 15 min post-test session. Data analysis reflected a marked improvement in mood and social behaviour and a significant decrease in non-social behaviour. The study outcomes demonstrate that familiar group singing positively affects the mood and social behaviour of residents with dementia experiencing sundowning, affirming the valuable role music therapists play in facilitating quality aged care.