In considering the cost of different forms of care for patients with mental deficiency there is a tendency to compare crude averages. Mental deficiency has many manifestations and occurs in people of widely differing degrees of physical ability and behaviour so that comparisons which do not take this into account are not meaningful. An attempt has been made to assess the cost of hospital care for different groups of patients and to show that there is a wide variation in the costs which arc normally expressed as an average. This average is often used to compare with the expenditure, say, in a hostel, when in fact those patients suitable for hostel care are at the lower end of the cost range. Again, whilst everyone would like to see patients dealt with in small groups, the extra cost this would involve is often overlooked. The analysis presented helps to clarify points such as these.