At times a clinician must describe the size of a lesion when measurement presents difficulties. A 'guestimate' may then be recorded. We tested the accuracy of forensic clinicians (10 physicians and 1 nurse) in guestimating the sizes of ten test objects, comparing their performance with 13 individuals from other professions. The participants were permitted to handle the objects but not to use a measuring device. For three of the objects (a balloon, a scratch and a coin) the entire sample of participants significantly overestimated size, by 13-22%. Both participant groups overestimated the size of a scratch, the clinicians being more accurate but not significantly so. Guestimates should be avoided unless the use of a ruler or tape measure will be against the interests of the client.