OBJECTIVE: Acceptance and commitment therapy, mindfulness-based cognitive therapy, and problem-solving therapy are types of cognitive-behavioural therapy (CBT) group that improve physical and mental health in chronic pain or cancer. However, dropout is high due to group demands alongside physical impairments. Motivational interviewing (MI) is a well-evidenced means of enhancing treatment adherence. Few studies have investigated MI as an adjunct to CBT in cancer or chronic pain, and none have established the minimum MI duration required for adherence improvement. This study evaluated minimal-duration MI to improve adherence in three CBT group types for cancer and chronic pain. METHODS: In a cohort study of 99 cancer and chronic pain patients, 47 were given a 10- to 15-min structured MI telephone intervention (MI-call) after the first session. The remaining 52 received a CBT group without MI (no-MI). RESULTS: Odds of completing group CBTs were five times greater for patients in the MI-call cohort versus no-MI. Effects remained when controlling for age, gender, diagnosis, group type, and baseline quality of life. The MI-call cohort attended one extra session per patient compared to no-MI, controlling for age, gender, and diagnosis. CONCLUSIONS: A brief MI telephone intervention may improve adherence to group CBTs in cancer and chronic pain. PRACTITIONER POINTS: A brief motivational interviewing (MI) telephone intervention may reduce dropout from group cognitive-behavioural therapies (CBTs) for cancer and chronic pain patients when administered after the first group session in routine care. Recipients of this intervention were five times more likely to complete a group CBT programme than those who did not receive it. Therefore, a minimal-dose MI intervention can have clinically important effects on dropout in group CBTs for patients with long-term conditions. It is unclear whether this intervention would also result in greater outcome improvements.