Difficulties surrounding the meaning of insight in psychiatry have led to neglect of its assessment in clinical practice. A study is described in which an insight scale was constructed and tested in a sample of 43 patients suffering from schizophrenia or depression. The results showed that the insight instrument was able to provide a qualitative and quantitative assessment of insight. Qualitative features brought out by the scale included patients' perception of changes within themselves and within their environment, their recognition of being ill and their acknowledgment of needing help. Quantitative measurements showed that there was a significant improvement in the level of insight over the period of hospital admission. In addition, scores obtained on the insight scale were inversely correlated with the severity of the patients' disorder, though these correlations occurred at different times in relation to the different disorders. We thus postulate that other factors (such as intellect, past experience, personality etc.) must be involved in modulating the expression of insight. Although the insight instrument needs refining, we suggest that it may be a useful tool in the further exploration of insight in clinical practice.