Nucleolar organizer regions (NORs) are genes coding for the ribosomal RNA; they also induce the formation of the nucleolus at interphase. Transcriptionally active NORs can be visualized in histological sections with a silver colloid method, allowing direct counting of these structures (so-called AgNORs). Seven superficial spreading melanomas with nodule (i.e., melanomas containing both a radial and a vertical growth phase) were studied with the technique. The nuclear AgNOR counts (mean +/- SEM) were 5.44 +/- 1.70 for the radial growth phase and 7.65 +/- 2.35 for the vertical growth phase (P less than 0.01). This difference in mean nuclear AgNOR numbers may be related to other known differences in the biological behavior of the two growth phases.