Urinary infections, with a spectrum from covert bacteriuria to severe pyelonephritis, commonly complicate pregnancy. Serious infections follow untreated silent bacteriuria in a fourth of cases, and routine screening can be justified in high-risk populations, particularly in those women from lower socioeconomic strata. Despite an initial salutary response to a number of antimicrobial regimens, covert bacteriuria recurs in one-third of treated women whose risk of pyelonephritis again is at 25%. Acute cystitis may be unrelated to these other infections and responds readily to a number of regimens; however, single-dose therapy is not recommended because early pyelonephritis can be mistaken for uncomplicated cystitis. Pyelonephritis is the most common severe bacterial infection complicating pregnancy. These women are frequently quite ill, and hospitalization is recommended. Since 85-90% respond within 72 h to intravenous fluids and antimicrobials, continued fever and evidence for sepsis should prompt a search for underlying obstruction. Perhaps 20% of women with severe pyelonephritis develop complications that include septic shock syndrome or its presumed variants. These latter include renal dysfunction, haemolysis and thrombocytopenia, and pulmonary capillary injury. In most of these women, continued fluid and antimicrobial therapy result in a salutary outcome, but there is occasional maternal morbidity.