OBJECTIVES: In October 2000, the ICD-9 diagnosis code 596.51 “Hypertonicity of Bladder” was updated to include “Overactive Bladder” (OAB). Previously, no specific code identified OAB. To determine implications for outcomes estimates, this study examined correspondence between suggested ICD-9 coding for OAB before the update and use of codes in claims data. METHODS: Data were drawn from medical claims of enrollees aged ≥18 years of a large US health plan. Fifteen ICD-9 diagnosis codes that might indicate OAB were identified through literature review and clinical consultation, and examined for agreement between sources. Possible OAB patients were identified if an OAB ICD-9 code appeared in claims from August 31, 1999–March 1, 2000. RESULTS: Sources agreed completely on five of 15 suggested ICD-9 codes. Of 33,290 patients with suggested codes, 86% had 1 of the 5 codes, and only 1% had 596.51 “Hypertonicity of Bladder” before the added OAB description. Most commonly used were “Urinary Frequency” (788.41; 48% of possible patients), “Unspecified Urinary Incontinence” (788.30; 19%), “Nocturia” (788.43; 15%), “Urge Incontinence” (788.31; 8%), and “Mixed Incontinence, Urge and Stress” (788.33; 5%). Proportion of subjects with the 5 codes differed by geographic region (p<0.0001), but not by gender (c2=3.30, p=0.07). Of 15 codes, “Nocturia” (788.43) was the only code with a greater proportion (74%) of men. Use of the five primary codes showed a linear trend (p<0.0001) increasing from 81% in ages 18–35 to 91% in ages ≥75. CONCLUSIONS: Before the ICD-9 code update, five codes accounted for most possible OAB patients and few were coded with 596.51. We expect claims data following the October 2000 implementation will gradually shift to the updated OAB code. Research should include 596.51 and the other five codes because coding practices may change and demographic coding differences could have an impact on outcomes estimates.