The prognosis of stage I pressure ulcers cannot be predicted; therefore, nursing interventions for preventing their deterioration have not been clearly established. This study describes the clinical course of stage I pressure ulcers and prospectively investigates the factors related to their deterioration. Thirty-one stage I pressure ulcers in 30 patients in a long-term care facility were studied, and morphological changes were assessed every day until the ulcers healed or deteriorated. The physiological changes were assessed by ultrasonography and thermography. Twenty ulcers healed, and 11 deteriorated. The characteristics of deterioration were as follows: (1) double erythema; (2) non blanchable erythema across the whole area determined by glass plate compression; (3) erythema away from the tip of the bony prominence; and (4) expanding erythema on the following day. We analysed the sensitivity, specificity, positive predictive value, negative predictive value and positive likelihood ratio for the diagnostic utility of the indicators of deterioration double erythema and distance from the tip of bony prominence, which can be instantly assessed without the use of any special device. The values were 36·4%, 95·0%, 80·0%, 73·1% and 7·28, respectively. These results suggest that clinicians can predict the prognosis of stage I pressure ulcers by initial assessment and provide appropriate care based on the assessment.