Clusters of cells in cytologic preparations from the urinary tract have been commented on repeatedly. To determine their significance we studied 88 patients with documented papillary carcinomas, Grades I-IV, who had cytologic examination of their urine. We defined two distinct cluster types, "rounded", and "papillary", which were easily distinguishable from artifactual excoriated cell aggregates. By cytologic techniques 64 per cent of the specimens were positive and 18 per cent suspicious for malignancy, with the lowest sensitivity for Grade I tumors, and the highest for Grade IV tumors. Clusters were observed in 39 per cent of the cases, and also were more common in the most undifferentiated tumors. However, many cases which were cytologically positive had no clusters, and in only one case were clusters present without other cytologic features of malignancy. It appears that the presence of cell clusters is not a more sensitive indicator of papillary lesions than usual cytologic criteria applied to single cells. Clusters may indicate a higher grade tumor.