The Discomfort Scale--Dementia of Alzheimer Type (DS-DAT) is an instrument to assess discomfort in patients with severe dementia by observing patients' behaviour during five minutes. Trained nursing home physicians collected data of 662 pneumonia patients with dementia. The data were used to test the validity of a Dutch translation of the DS-DAT as a measure of discomfort. Internal consistency of the nine-item instrument was favourable (Cronbach's alpha ranged between 0.82 and 0.84 for different times during and following the pneumonia), and factor analyses indicated measurement of a single concept. Furthermore, as hypothesised, discomfort appeared moderately associated with acute illness. Nursing home physicians' assessment of DS-DAT scores therefore appeared a suitable method to assess discomfort in severely demented psychogeriatric nursing home patients for research purposes. For use in individual patients, results are probably too strongly affected by the chosen time fluctuations of assessment. For this purpose, the current rater assessment instructions should be adapted and tested.