Twenty-three cases of cystosarcoma phyllodes of the breast were studied clinically and by fine-needle aspiration (FNA) cytology as well as by tumor histology. The median patient age was 52 yr. The total actuarial survival at 5 yr was 90%, and the recurrence-free survival was 70%. A combination of stromal elements (tissue fragments and/or single spindle cells) and epithelial tissue was required to make the diagnosis on smear material. In FNA smears, stromal elements were absent in two cases, and ductal elements were not seen in three. Thus, the dimorphic pattern essential to this diagnosis was not present in five (22%) cases. An evaluation of tumor factors reported to be of prognostic significance was performed on histologic sections of resected tumors. Comparison of these data with FNA results indicates that little prognostic information can be obtained from FNA alone in most cases.