The distribution of mannose-resistant (MRHA) and mannose-sensitive (MSHA) fimbrial haemagglutinins was examined in 482 strains of Escherichia coli isolated from 390 adult women and 45 pregnant mothers with a variety of urinary tract infections (UTI), and from 47 healthy controls. The proportion of MRHA strains was significantly higher in patients with symptomatic UTI (75%) than in women with non-significant bacteriuria (30%, p less than 0.001), pregnant women with asymptomatic UTI (34%, p less than 0.0001) and healthy controls (0%). The proportion of MSHA strains was significantly lower in patients with symptomatic UTI (22%) than in women with non-significant bacteriuria (46%, p less than 0.001) and pregnant women with asymptomatic UTI (52%, p less than 0.01). Only 17% of the strains from healthy controls had MSHA activity. In pregnant women with UTI, whether this was symptomatic or asymptomatic, there was a significant association between infection with MRHA strains of E. coli and a past history of UTI. Thus, in a pregnant woman with an infection and a past history of UTI there is a seven-fold greater chance that this infection is due to an MRHA-bearing organism than in pregnant women without such a history. There was also a significant association between MRHA organisms and symptomatic infection. The risk of symptomatic patients having an infection with an MRHA strain is six times greater than that for a patient with a covert infection.