Background: Monitoring socioeconomic inequalities in sexual health service use is needed to ensure equitable service provision. There are limitations to current methods, particularly when applied to young people. The present study examined the validity and added value of combining ACORN, a commercial tool, with an established deprivation index to improve monitoring of inequalities in the National Chlamydia Screening Programme (NCSP) in England. Methods: ACORN categories and the Index of Multiple Deprivation 2004 (IMD 2004) quintiles were linked to England population estimates and 145 975 records from the NCSP. Results: ACORN’s validity was demonstrated by moderate agreement between IMD 2004 and ACORN in segmenting the population and consistent gradients in the relative risk of chlamydia positivity between the most and least deprived areas assessed by IMD 2004 (relative risk (RR) 1.32; 95% confidence interval (CI) 1.23, 1.40) and ACORN (RR 1.32; 95% CI 1.25, 1.40). ACORN’s fine level of geographic resolution demonstrated that the NCSP was reaching the most deprived neighbourhoods within larger areas classified overall as deprived by IMD 2004. Within the most deprived IMD 2004 quintile, areas classified as most deprived by ACORN had higher chlamydia screening coverage (3.15%; 95% CI 3.11, 3.19%) than the least deprived (1.81%; 95% CI 1.56, 2.08%). Furthermore, ACORN identified highest screening coverage (9.2%) in areas classified as ‘communal housing.’ Conclusions: These findings illustrate that a commercial geodemographic tool in combination with an established deprivation index may overcome limitations in monitoring inequalities in sexual health service use. ACORN had value as a supplementary deprivation marker for monitoring inequalities in chlamydia screening in England.