In December 1994 we completed a longitudinal, cohort, vaccine efficacy trial and the results were presented at last years APS/SPR annual meeting. We now report efficacy following household exposure in the same trial. During the 3.5 year study period 10,271 vaccinees (8532 DTP or DTaP; 1739 DT) and their family members were closely monitored for cough illnesses. Cough illnesses of≥ 7 days duration were studied (culture, PCR and pertussis antibodies [PT, FHA, pertactin, fimbriae-2 and agglutinins]) and vaccinees or family members with cough illnesses ≥ 14 days duration were seen by a Central Investigator. Efficacy estimates for both DTP and DTaP varied significantly using different definitions for primary, secondary and non-cases; exposure periods; and the inclusion or exclusion of non-cases who received macrolide antibiotics or trimethoprim- sulfamethoxazole prophylacticly. Using laboratory confirmed B. pertussis infections with ≥7 days of cough in primary and secondary cases, a 7 to 42 day observation period and the exclusion of non-cases who received prophylactic antimicrobials the efficacy of DTP was 84% (95% CI=65-93) and DTaP was 58% (95%CI=30-75). The efficacy against laboratory confirmed typical pertussis (≥ 21 days of cough with either paroxysms, whoop or posttussive vomiting) was 94% (95% CI=77-99) and 86% (95% CI=62-95) for DTP and DTaP respectively. The efficacy against any cough illness (with or without laboratory confirmation) was 54% (95%CI=32-69) and 38% (95%CI=13-56) for DTP and DTaP respectively. In summary this household contact substudy within our cohort study, with active, investigator generated, surveillance (which was not done in other recent trials) was a severe test of vaccine efficacy. When compared with our cohort results the efficacy of DTP against laboratory confirmed illness of ≥7 days duration as well as typical pertussis was similar in this household contact analysis; DTaP was also highly efficacious against typical pertussis, whereas efficacy aginst illness of ≥ 7 days was less after household exposure than after exposure in the community.