Aims: To define clinically relevant cut-off points for severe fear of self-injecting (FSI) and self-testing (FST) (phobia) in insulin-treated patients with diabetes, and to estimate the magnitude of these phobias in our research population. Methods: FSI and FST were assessed in a cross-sectional survey using the Diabetes Fear of Injecting and Self-testing Questionnaire (D-FISQ). A sample of 24 insulin-treated adult diabetic patients was selected from the high-scorers on FSI and/or FST (≥ 95th percentile). FSI and FST were re-assessed, after which patients participated in a behavioural avoidance test (BAT), thereby determining the current level of avoidance of either self-injecting or self-testing. FSI and FST scores were linked to the outcome of the BATs. Cut-off scores for severe FSI/FST were determined and extrapolated to the total study population (n = 1275). Results: Seven patients participated in the self-injecting BAT: two patients refused to perform an extra injection. In the self-testing BAT (n = 17) four patients declined to perform the extra blood glucose self-test. Extrapolation of FSI and FST cut-off scores to the total research population showed that 0.2-1.3% of the population scored in the severe FSI range. In FST, 0.6-0.8% of the total study population obtained scores in the cut-off range. Conclusions: Severe FSI and FST, characterized by emotional distress and avoidance behaviour, seems to occur in a small group of insulin-treated patients with diabetes. The D-FISQ can be of use to health care professionals (physicians, nurse specialists) in quickly providing valuable information on levels of FSI and FST in diabetes patients.