Due to respiration, many tumours in the thorax and abdomen may move as much as 3 cm peak-to-peak during radiation treatment. To mitigate motion-induced irradiation of normal lung tissue, clinics have employed external markers to gate the treatment beam. This technique assumes that the correlation between the external surface and the internal tumour position remains constant inter-fractionally and intra-fractionally. In this work, a study has been performed to assess the validity of this correlation assumption for external surface based gated radiotherapy, by measuring the residual tumour motion within a gating window. Eight lung patients with implanted fiducial markers were studied at the NTT Hospital in Sapporo, Japan. Synchronized internal marker positions and external abdominal surface positions were measured during the entire course of treatment. Stereoscopic imaging was used to find the internal markers in four dimensions. The data were used retrospectively to assess conventional external surrogate respiratory-gated treatment. Both amplitude- and phase-based gating methods were investigated. For each method, three gating windows were investigated, each giving 40%, 30% and 20% duty cycle, respectively. The residual motion of the internal marker within these six gating windows was calculated. The beam-to-beam variation and day-to-day variation in the residual motion were calculated for both gating modalities. We found that the residual motion (95th percentile) was between 0.7 and 5.8 mm, 0.8 and 6.0 mm, and 0.9 and 6.2 mm for 20%, 30% and 40% duty cycle windows, respectively. Five of the eight patients showed less residual motion with amplitude-based gating than with phase-based gating. Large fluctuations (>300%) were seen in the residual motion between some beams. Overall, the mean beam-to-beam variation was 37% and 42% from the previous treatment beam for amplitude- and phase-based gating, respectively. The day-to-day variation was 29% and 34% from the previous day for amplitude- and phase-based gating, respectively. Although gating reduced the total tumour motion, the residual motion behaved unpredictably. Residual motion during treatment could exceed that which might have been considered in the treatment plan. Treatment margins that account for motion should be individualized and daily imaging should be performed to ensure that the residual motion is not exceeding the planned motion on a given day.