Background: Pelvic inflammatory disease (PID) is difficult to define and diagnose; therefore, a standardized methodology for identifying and monitoring PID diagnoses is required. We estimated the rate of PID in general practice in England, and investigated variations by definition of PID, time, age, and geographical area. Methods: We analyzed the United Kingdom General Practice Research Database between 2000 and 2008. Definitions of "definite," "probable," and "possible" PID among female patients (aged 16 to 44 years) were determined according to medical codes that denoted diagnoses or symptoms indicative of PID. Diagnoses rates were calculated per 100,000 person-years (py). Trends were assessed using Poisson regression. Results: The rate of clinical PID diagnoses was 281/100,000 py (95% confidence interval [95% CI]: 277-286) for definite cases; 326/100,000 py (95% CI: 321-331) for definite and probable cases; and 1117/100,000 py (95% CI: 1107-1126) for definite, probable, and possible cases. During 2000 to 2008, the rate of definite/probable PID decreased by 10.4% per year (95% CI: 9.7-11.1; P < 0.001). Rates declined in all areas and among all age groups with greatest decline in women aged 16 to 19 years. Meanwhile, the rate of possible PID increased. CONCLUSIONS:: The definition of PID used has a major effect on the rate and trends over time. There was heterogeneity in rates of definite/probable PID by age and region, but homogeneity with regard to a trend of declining rates. Ongoing monitoring of PID diagnoses, with standard case definitions, will contribute to the evaluation of chlamydia screening in England.