ABSTRACTBackground:To examine the effects of complex cognitive (mahjong) and physical (Tai Chi) activities on dementia severity in nursing home residents with dementia.Methods:Cluster-randomized open-label controlled design. 110 residents were randomized by nursing home into three conditions: mahjong, Tai Chi, and simple handicrafts (control). Activities were conducted three times a week for 12 weeks. Clinical Dementia Rating (CDR) was taken at 0 (baseline), 3 (post-treatment), 6, and 9 months. The outcome measure was CDR sum-of-box, which is a composite measure of both cognitive and functional deterioration in dementia.Results:Intent-to-treat analyses were performed using multilevel regression models. Apolipoprotein E ε4 allele and education were included as covariates. Neither treatments had effects on the cognitive and functional components of the CDR, but mahjong had a significant interaction with time on the CDR sum-of-box total, suggesting a slower rate of global deterioration in the mahjong group as compared with the control group.Conclusions:Mahjong led to a gradual improvement in global functioning and a slightly slower rate of dementia progression over time. The effect was generalized and was not specific to cognition or daily functioning.