Mycosis fungoides (MF) and Sézary's syndrome are cutaneous T cell lymphomas, characterized by the presence of lymphoid cells with deeply indented nuclei (CMC) in the infiltrate. In order to find objective criteria for the diagnosis of early MF involvement of lymph nodes from patients with MF, we performed morphometric analysis of lymphoid cells in lymph node cell suspensions measuring the degree of nuclear indentation as expressed by the nuclear contour index (NCI). Statistical discriminant analysis was used to analyze the differences in the NCI histograms between lymph nodes without and with MF involvement and to select the most discriminating parameters for diagnostic classification. Using a training set of 6 lymph nodes from patients with unrelated diseases and 8 lymph nodes from patients involved by cutaneous T cell lymphomas, the mean and standard deviation of the NCI histograms were selected as the most discriminating parameters. All lymph nodes from the training set were assigned to the correct diagnostic classification group with a probability over 90%. The predictive value of the morphometric classification was tested on a set of 12 enlarged lymph nodes from patients with MF. The histological diagnosis was used as a reference. In 10 cases the morphometric classification was identical to the histological classification, whereas in two cases (1 classified as positive, 1 as negative) a disagreement was found. It is concluded that morphometry of lymphoid cells can contribute substantially to the diagnosis of early MF involvement in lymph nodes.