PURPOSE: To determine whether treatment with nitrofurantoin in women with urinary tract infection (UTI) and renal impairment in primary care is associated with a higher risk of ineffectiveness and/or serious adverse events than in women without renl impairment. METHODS: A cohort of 21,317 women treated with nitrofurantoin and a cohort of 7,926 women treated with trimethoprim, identified from the Pharmo Record Linkage System, were analysed. The primary outcome was ineffectiveness of treatment of nitrofurantoin defined as the start of a second antibacterial within 1 month after the start of nitrofurantoin. The secondary outcome was the occurrence of serious adverse events of nitrofurantoin leading to hospitalization within 90 days. A cohort of trimethoprim users was used to determine if the associations found for nitrofurantoin were mainly related to nitrofurantoin itself. The association between renal impairment and the risk of these outcomes was determined with Cox regression and expressed as hazard ratios (HRs). RESULTS: Overall, the incidence density for ineffectiveness was 5.4 per 1,000 person-days, and moderate renal impairment was not associated with ineffective treatment [HR 1.1, 95 % confidence interval (CI) 0.74-1.51]. The overall incidence density for adverse events was 0.02 per 1,000 person-days. In patients with renal impairment (<50 ml/min/1.73 m(2)) the risk of pulmonary adverse events leading to hospitalization was significantly increased (HR 4.1, 95 % CI 1.31-13.09) CONCLUSIONS:Nitrofurantoin treatment was not associated with a higher risk of ineffectiveness in women with UTI and moderate renal impairment (30-50 ml/min/1.73m(2)). However, we did find a significant association between renal impairment (<50 ml/min/1.73 m(2)) andpulmonary adverse events leading to hospitalization.