Reports suggest that it is possible and useful to classify T1 (Stage A) carcinomas of the urinary bladder into subgroups dependent on the depth of invasion as defined by the muscularis mucosae (MM). In an attempt to evaluate the reproducibility of these findings, we reviewed the slides from 77 cases of T1 bladder cancer diagnosed in 1983 in Iowa residents. The cases were identified through a statewide cancer registry. Slides and pathology reports were obtained from the original laboratories. Two pathologists independently evaluated each case, most (74 cases) of which were transurethral resections, for the presence or absence of MM and three levels of invasion (lamina propria, MM, and submucosa). Disagreements were resolved by simultaneous review resulting in a consensus diagnosis. Because of the population-based source, tissue had been obtained by multiple urologists and processed in multiple laboratories, resulting in noticeable variation in quality of material. Interobserver agreement for the level of invasion was poor between the two pathologists (kappa = 0.22; 95% C.I. = 0.08-0.36). Consensus resulted in 34 cases (44.2%) invasive to the lamina propria, 23 (29.9%) to the MM, and 11 (14.3%) to the submucosa. The level was indeterminate in nine (11.7%). All of the 77 cases were followed until death or 1993-95, with 57 patients dying during this interval. There were no significant differences in survival for each level of invasion (lamina propria, MM, submucosa) and the indeterminate cases, as determined by either consensus or individual observer evaluation. Our findings suggest that microstaging of early invasive bladder cancer is technically difficult and, at least in cases derived from various urologists and laboratories, does not yield a prognostically significant separation.