Quantitative electron microscopy was used to measure lymphocyte nuclear contours in skin biopsies from 109 patients, 77 with benign disorders, 16 with early mycosis fungoides (MF), and 16 with controversial lesions. A nuclear contour index (NCI) was calculated on electron micrographs by dividing the nuclear profile circumference by the square root of the nuclear area. Significant differences were found between the group mean NCI for MF (NCI = 6.1 +/- 0.1) and for the benign group (NCI = 4.6 to 5.4 +/- 0.1; P less than 0.005). The group mean NCI for the controversial cases was significantly lower than that of the MF group (P less than 0.05). It is suggested that a definite electron microscopic diagnosis of MF requires first a patient mean NCI of 6.1 or more, and second, at least 6% of lymphocytes with an NCI of 9 or more. These criteria give a false positive rate of 3% in benign and controversial disorders and a 50% false negative rate for early MF. Only lymphocytes with an NCI of 16 or more appear specific for MF (or Sezary's syndrome), but are not found in all cases.