The purpose of this paper was to compare the efficacy of a single dose of 3 g of fosfomycin to that of a 7-day regimen of amoxicillin-clavulanate in the treatment of asymptomatic bacteriuria during pregnancy. A randomised, prospective, interventional, analytical, longitudinal study was undertaken, in which the efficacy of two antibiotic regimens (one short and the other long) in the treatment of pregnant women with asymptomatic bacteriuria is compared. One hundred and nine patients were randomly assigned to two groups: 56 were treated with amoxicillin-clavulanate and 53 with fosfomycin. The two groups were similar in terms of co-morbidity, treatments received during pregnancy, obstetric, gynaecological and surgical history and laboratory data. The efficacy of the two regimens was similar and the eradication rate was over 80% in both groups (P = 0.720) (relative risk [RR] 1.195, 95% confidence interval [CI]: 0.451-3.165). The number of reinfections was greater in the amoxicillin-clavulanate group (P = 0.045). The secondary effects were lower in the fosfomycin group (P = 0.008). There were no significant differences in the number of persistences (P = 0.39), development of symptomatic urinary infections (P = 0.319) or recurrences (P = 0.96). Treatment with a single dose of fosfomycin is as effective as the standard course of treatment with amoxicillin-clavulanate and may be preferable due to its simpler administration and the smaller number of reinfections.