Purpose To improve understanding about; (1) the validity of the Distress Thermometer (DT) as a measure of changes in distress after a cancer helpline call, (2) the impact of a helpline call on callers' distress, (3) caller and helpline nurses’ comfort with use of the DT, and (4) the extent to which DT scores over the critical threshold, are associated with referral to internal support services for follow-up psychosocial care. Methods Callers (people diagnosed with cancer and their family/friends: N = 100) completed a questionnaire that included DT ratings (three time-points), the Depression Anxiety and Stress Scale-21 (DASS-21) and measures of comfort with the DT tool. Nurses recorded referrals to internal services and their comfort in using the DT in each call. Results The DT correlated with the DASS-21 depression (r = 0.45, p < 0.001), anxiety (r = 0.56, p < 0.001) and stress (r = 0.64, p < 0.001) subscales demonstrating validity. Callers' self-rated distress was significantly lower after the call, regardless of gender or caller type (F(2, 97) = 63.67, p < 0.01, partial eta squared = 0.57). Over 74% of people diagnosed with cancer, 80% family/friends and 89.3% of nurses felt comfortable with DT use. Only 16% of participants were referred on to follow-up internal support services despite 90% of people with cancer and 75% of family/friends' DT scores’ suggesting they required follow-up care. Conclusions The DT is a valid and acceptable tool for use by cancer helplines. Improved documentation of referrals is required to better understand referral patterns.