AIM: To assess determinants of treatment failure after antimicrobial therapy of urinary tract infections in women. METHODS: In primary care 16 703 Dutch women who received a first course (3, 5 or 7 days) of trimethoprim, nitrofurantoin or norfloxacn between 1 January 1992 through 31 December 1997 and who were between 15 and 65 years old at the day of first use were selected. Failure of the initial treatment was defined as a further prescription for one of these three antibiotics or for cotrimoxazole, amoxicillin, ciprofloxacin or ofloxacin, within 31 days after the end of the initial therapy. RESULTS: Treatment failure rate was 14.4% in patients treated with trimethoprim and nitrofurantoin and 9.6% in patients treated with norfloxacin. A multivariate analysis showed that 5 days'[RR(NIT) 0.67, 95% confidence interval (CI) 0.57, 0.82, RR(TRI) 0.82, 95% CI 0.73, 0.91] and 7 days’ (RR(NIT) 0.64, 95% CI 0.53, 0.77, RR(TRI) 0.85, 95% CI 0.71, 1.02) trimethoprim and nitrofurantoin treatment appeared tobe more effective than a 3-day treatment (reference category). Other factors increasing treatment failure rates were the age of the patient, the year of therapy and previous hospitalization. CONCLUSIONS: It may be concluded that 3-day courses of nitrofurantoin and trimethoprim are less effective than 5- and 7-day courses in the treatment of uncomplicated urinary tract infections in women.