We studied the relationship between sleep quality and bed surface firmness. Nine men were investigated, sleeping in their homes for at least 5 consecutive nights on a soft and a more firm mattress using a sensor pad placed under the mattress and a solid-state recording device. The subjective feeling of sleep quality did not always agree with the recorded sleep data. The difference was most marked when changing from the subject's own to one of the test mattresses. For the same subject the results were reproducible between nights provided there were no external disturbing factors. Four of the 9 subjects slept significantly better on the softer of the two mattresses and 2 on the hard mattress. The difference in sleep quality observed among the subjects tested makes it necessary to relate the results to the same person rather than considering a whole group as an entity. The adaptation period for a new sleep surface extended to many days.