A 51-year-old man presented with acute cystitis at a time when his wife was admitted with symptoms of acute pyelonephritis. Before the man developed his symptoms his wife had complained of urinary frequency and dysuria for a 2-week period. Two weeks after a 10-day course with trimethoprim he experienced a relapse manifesting as acute febrile pyelonephritis. The strains of Escherichia coli isolated from the urine of both patients belonged to the same serotype O51, O117:K1, were non-haemolytic, produced aerobactin, expressed P-fimbriae, and showed identical antibiotic susceptibility pattern. The findings strongly suggest that the E. coli strain might have been sexually transmitted from the wife to her husband.