Background Activity levels remain low in care homes, but activity engagement can enhance residents' quality of life. Aim This study aimed to assess an occupational therapy programme designed to enable care home staff to increase activity provision. Method A cluster randomised controlled trial with blinded assessment of outcome was conducted. A total of 210 residents with dementia in 16 care homes were recruited. Intervention homes received the programme, and control homes were provided usual care. Primary outcome is quality of life; secondary measures are dependency, challenging behaviour, depression, anxiety, severity of dementia and number and type of medication. Results Quality of life decreased overall with statistically significant change in staff ratings (p < 0.001). At follow‐up, staff‐rated quality of life was slightly lower in the intervention group (mean difference in staff ratings = −1.91, 95% CI −3.39 to −0.43, p = 0.01). There were no significant differences between the two groups for self‐rated quality of life or secondary outcomes. Conclusion The results may be related to the following: wide variability in how the intervention was implemented, such as low staff attendance at the education and coaching sessions, and patchy provision of additional activities to residents; or the residents' severity of dementia or the choice of outcome measures. Future studies need to pay more attention to process measures such as implementation and fidelity strategies, and outcome measures that better capture the focus of the intervention such as level of engagement and activity. Copyright © 2013 John Wiley & Sons, Ltd.