A prospective validation study was conducted in 171 consenting patients from oncology and palliative care outpatient clinics to validate the Distress Thermometer (DT) against the Hospital Anxiety and Depression Scale (HADS), General Health Questionnaire-12 (GHQ-12) and Brief Symptom Inventory-18 (BSI-18) at baseline, four weeks and eight weeks. Receiver Operating Characteristic analysis was used to examine the sensitivity and specificity of the DT scores against the clinically significant cut-off scores of the criterion measures reporting 95% confidence intervals. Standardised response means were used to compare DT scores with criterion measures over time. For a cut-off of 4 vs 5, sensitivity against HADS was 79%, specificity 81%; against GHQ-12, sensitivity was 63%, specificity 83%; and against BSI-18, sensitivity was 88%, specificity 74%. At both four and eight weeks, DT scores tended to change significantly in the same direction as the criterion measures. Ninety-five percent of patients found completing the DT acceptable. The DT is valid and acceptable for use as a rapid screening instrument for patients in the UK with cancer. Our results indicate that it can be used to monitor change in psychological distress over time, but further work is needed to confirm this.