The density and distribution of lymphocytes infiltrating the vertical growth phase of primary cutaneous melanomas has been suggested by several studies to be of prognostic significance. However, few pathologists comment on tumor-infiltrating lymphocytes (TILs), and there is the perception that the assessment of TILs is subject to great interobserver variability. We studied interobserver agreement on the categorization of TILs; 20 cases of primary cutaneous malignant melanoma with a vertical growth phase component were circulated among 3 pathologists and 3 dermatologists. For each case, TILs were classified as brisk, nonbrisk, or absent according to Clark. Only 1 pathologist (a dermatopathologist) was familiar with the classification of TILs. Observers were given written guidelines and a brief tutorial before their examination of the slides. Our results show that with little instruction, overall agreement among observers was good (kappa values, 0.6 or more), especially among pathologists (kappa values, > 0.7). Three observers had excellent agreement among each other (kappa values, > 0.75). These findings suggest that the categorization of TILs can be easily taught and can be applied with an acceptable level of reproducibility in routine diagnostic practice.