OBJECTIVE: To explore clinicians' knowledge and consideration of radiation, in relation to their referral practice and use of referral guidelines for imaging. METHODS: A questionnaire was handed out to 213 clinicians in Norway; all responded: 77 general practitioners, 71 hospital physicians and 65 non-physicians (55 manual physiotherapists, 10 chiropractors). Questions concerned weighting of radiation dose, guideline use, referrals unlikely to affect treatment, doses from imaging procedures, ranking of imaging as radiation source, and deterministic and stochastic effects. For radiation knowledge, a total score was aggregated. RESULTS: The mean radiation knowledge score was 30.4/71. Most respondents underestimated doses from high-dose imaging, e.g., barium enema (94.7%), chest CT (57.7%) and abdominal CT (52.7%). Limited radiation knowledge was not compensated by using guidelines. Only 20% of physicians and 72% of non-physicians used referral guidelines. Non-physicians weighted radiation dose as being more important than physicians when referring; they also reported fewer referrals as being unlikely to affect treatment. Such referrals and not using guidelines were related to lower weighting of radiation dose but not to radiation knowledge. CONCLUSION: Limited radiation knowledge and guideline use indicate suboptimal justification of referrals. When justifying imaging, weighting of radiation dose may play a larger role than detailed radiation knowledge.