BACKGROUND: Blood-injection-injury (B-I-I) phobia is capable of producing inaccurate hypoxic challenge testing results due to anxiety-induced hyperventilation. CASE REPORT: A 69-yr-old woman with a history of hypersensitivity pneumonitis, restrictive spirometry, exercise desaturation requiring supplementary oxygen on mobilizing, reduced DLco, and B-I-I phobia was referred for hypoxic challenge testing (HCT) to assess in-flight oxygen requirements. HCT was performed by breathing a 15% F(I)o(2) gas mixture, simulating the available oxygen in ambient air onboard aircraft pressurized to an equivalent altitude of 8000 ft. S(p)o(2) fell to a nadir value of 81% during HCT, although it rapidly increased to 89% during the first of two attempts at blood gas sampling. A resultant blood gas sample showed an acceptable Po-2 outside the criteria for recommending in-flight oxygen and a reduced Pco(2). Entering the nadir S(p)o(2) value into the Severinghaus equation gives an estimated arterial Po-2 of 6 kPa (45 mmHg), which was felt to be more representative of resting values during HCT, and in-flight oxygen was recommended. DISCUSSION: While hyperventilation is an expected response to hypoxia, transient rises in S(p)o(2) coinciding with threat of injury are likely to be attributable to emotional stress-induced hyperventilation, characteristic of B-I-I specific phobia and expected during the anticipation of exteroceptive threat, even in normal subjects. In summary, should excessive hyperventilation be detected during HCT and coincide with transient increases in S(p)o(2), HCT should be repeated using S(p)o(2) only as a guide to the level of hypoxemia, and Spot maintained using supplementary oxygen in accordance with alternative methods described in guidelines.