We assessed the quality of evidence for the use of erythromycin in preventing secondary transmission of pertussis to close contacts of primary cases. A literature search was undertaken and identified papers were reviewed critically. Thirteen original papers and 1 manuscript met the inclusion criteria for review (3 randomized controlled trials, 4 analytical studies and 7 descriptive studies). Evidence from both experimental and analytical studies showed little effect of the use of erythromycin in preventing secondary transmission. Its effect is at best modest when compared with the protection conferred by use of good quality whole cell vaccine. Three studies reported adverse events with erythromycin prophylaxis; these were mainly nausea, vomiting and abdominal pain. In countries where effective pertussis vaccines are in use, erythromycin use should be confined to close contacts of cases, particularly unimmunized children or partially immunized infants who would be most susceptible to the complications of pertussis, or adults who come into close contact with vulnerable children.