Objectives To determine predictors of parenteral antibiotic duration and the association between parenteral treatment duration and relapses in infants <3 months with bacteraemic urinary tract infection (UTI).

Design Multicentre retrospective cohort study.

Setting Eleven healthcare institutions across the USA.

Patients Infants <3 months of age with bacteraemic UTI, defined as the same pathogenic organism isolated from blood and urine.

Main outcome measures Duration of parenteral antibiotic therapy, relapsed UTI within 30 days.

Results The mean (±SD) duration of parenteral antibiotics for the 251 included infants was 7.8 days (±4 days), with considerable variability between institutions (mean range 5.5–12 days). Independent predictors of the duration of parenteral antibiotic therapy included (coefficient, 95% CI): age (−0.2 days, −0.3 days to −0.08 days, for each week older), year treated (−0.2 days, −0.4 to −0.03 days for each subsequent calendar year), male gender (0.9 days, 0.01 to 1.8 days), a positive repeat blood culture during acute treatment (3.5 days, 1.2–5.9 days) and a non-Escherichia coli organism (2.2 days, 0.8–3.6 days). No infants had a relapsed bacteraemic UTI. Six infants (2.4%) had a relapsed UTI (without bacteraemia). The duration of parenteral antibiotics did not differ between infants with and without a relapse (8.2 vs 7.8 days, p=0.81).

Conclusions Parenteral antibiotic treatment duration in young infants with bacteraemic UTI was variable and only minimally explained by measurable patient factors. Relapses were rare and were not associated with treatment duration. Shorter parenteral courses may be appropriate in some infants.