A modification of the single optotype Sheridan Gardiner test for pre-school children has been used to measure visual crowding. A significant 'crowding effect' has been found in children between the ages of 3 and 6 years with a general decrease in the effect over the pre-school years. The 'crowding' in 5-7 year olds is not significantly greater than that found in adults. No significant difference has been found in the extent of crowding using the test at 3 m rather than the normal 6 m viewing distance, although in general the test has been found to be practically easier with better co-operation at 3 m for the younger children. For these reasons, this particular test at 3 m would seem to be well suited to acuity testing in pre-school children, when an acuity equivalent to Snellen values is required.