OBJECTIVES: To investigate whether meningococcal C conjugate vaccine (MCCV) caused relapse in children with steroid-responsive nephrotic syndrome. DESIGN: A population-based study was conducted using an active surveillance system, developed to assess adverse events following vaccination, which linked hospital record information on relapses of nephrotic syndrome to community child health population MCCV data. An ecological study looking at hospital admissions for nephrotic syndrome in different age cohorts of children before and after the MCCV introductory campaign was also carried out. SETTINGS: South East England, and England and Wales. PATIENTS: 52 children having 162 relapses of nephrotic syndrome. Also, all hospital admissions of children aged 2-18 years with steroid-responsive nephrotic syndrome in England and Wales between 1995 and 2003, relating admissions to when MCCV was introduced in specific age cohorts. MAIN OUTCOME MEASURES AND ANALYSIS METHOD: Self-controlled case series analysis looking for increased risk of relapse following MCCV and changes in admission rates for nephrotic syndrome (incidence ratio) following the introduction of MCCV to different age cohorts of children. RESULTS: There was no increased risk of relapse following MCCV in the self-control case series, where a relative incidence of 0.95 (95\% confidence interval (CI) 0.61-1.47) was found in the 6-month post-vaccination period, or in the ecological study, which gave an incidence rate ratio of 1.05 (95\% CI 0.95 to 1.15) for the quarter when MCCV was introduced and the following two quarters. CONCLUSIONS: We found no association between MCCV and nephrotic syndrome, which is therefore not a contraindication to meningococcal vaccination.