Eighteen cases of transitional cell carcinoma (TCC) of the urinary bladder containing a micropapillary component (MPC) (> 90%, three cases; 50-90%, nine cases; < 50%, six cases) are presented. The patients' mean age was 66.6 years (range, 47-81 years) with a male predominance (male-to-female ratio of 5:1). The MPC was part of the surface noninvasive TCC in 16 cases and part of the invasive portion of the TCC in all 18 cases. Eight patients had metastases, each with a predominant (> or = 50%) MPC in the metastases; local recurrence was documented in one case, and the tumor was locally invasive into pelvic structures in three cases. Histologically, the surface MPC comprised slender, delicate filiform processes or small papillary clusters of tumor cells, whereas the deep MPC was composed of infiltrating tight clusters of micropapillary aggregates that were often present within lacunae. Vascular-lymphatic invasion was consistently present in the micropapillary areas. The cytologic features of the MPC were those of grade 3 TCC. A concurrent TCC in situ was identified in 10 cases, noninvasive papillary TCC component in all cases, and glandular differentiation of the invasive TCC in five cases. The initial stage at presentation was usually high stage: one patient with stage A, nine with stage B, six with stage C, and two with stage D. Follow-up data (mean, 44.8 months; range, 6-96 months) indicated that four patients were alive with disease, seven patients were dead of disease, and there was no evidence of disease in seven patients. In five cases, DNA ploidy analyzed by static image analysis showed nondiploid indices within the micropapillary TCC component, and in three cases the DNA index of the MPC (noninvasive, invasive, or at metastatic site) exceeded the DNA index of the noninvasive papillary TCC. In conclusion, the presence of a MPC in TCC is associated with high-grade and high-stage TCC with a tendency to vascular invasion. Our data suggest that a surface MPC in bladder biopsy specimens is a poor prognostic histologic feature and, if the biopsy does not contain muscularis propria, deeper biopsies should be recommended to determine the presence of muscle invasion.