Cancer registry problems in classifying invasive bladder cancer. A slide review of diagnostic pathologic tissue obtained from 364 bladder cancer cases, identified through the Iowa Surveillance, Epidemiology, and End Results (SEER) Program in 1983, classified 97 (26.6%) of these cases as invasive bladder cancers. These findings contrasted sharply with the Iowa SEER Program classification that coded 289 (79.4%) of these cases as invasive bladder cancers. These results were validated further by the hazard ratio of 4.54 (95% confidence interval, 2.57 to 8.03) among invasive relative to noninvasive bladder cancer cases when the slide review findings were used. In contrast, the hazard ratio was only 1.70 (95% confidence interval, 0.76 to 3.79) when the Iowa SEER Program findings were used. The traditional method used by the National Cancer Institute's SEER Program to deal with this problem is described and its implications are discussed. 