Fifty portable oxygen assessments, based on 6-min walking tests, were reviewed to produce guidelines for assessment and prescription of portable oxygen, and to evaluate factors predicting benefit. Patients with a portable cylinder were questioned about benefits/difficulties and usage of portable oxygen. A significant improvement on the oxygen compared to the air walk was found (median 9.7%) which correlated with the oxygen desaturation on the baseline walk (r = 0.51, P less than 0.05). A placebo effect was demonstrated using an air cylinder (median improvement 6.1%). There was no correlation between improvement in walking distance and decrease in visual analogue scale (VAS) score of breathlessness. We recommend standard portable oxygen assessments based on a series of 6-min walks. Improvements of 10% in walking distance or VAS score from the oxygen to the air walk would lead to the patient being offered a portable oxygen cylinder.