23 urological patients, primarily with underlying disease of the urinary tract, received a 10-day treatment with either pivmecillinam/pivampicillin or pivmecillinam alone. Combination therapy eradicated urinary pathogens in the urine at a 48 h control in 11/11 patients treated with the combination compared with 6/12 patients receiving pivmecillinam alone (p less than 0.05). Two of the treatment failures on pivmecillinam alone were caused by Enterobacteriaceae that were in vitro sensitive to mecillinam. A repeated course with the combination was successful in both of these patients. Apart from a rash seen in one patient in the combination group, no side effects were recorded.