Hepatitis A, varicella, and mumps are common in countries without universal immunization against these infections. Information about secondary household attack rates is limited. A prospective study was conducted in children < 16 y of age who had household contact with the above infections. History for previous infections and vaccination was obtained from the parents. Specific antibodies in the sera during enrolment and at the end of the longest incubation periods were determined. A total of 113 children who had household contact with cases of varicella, hepatitis A, or mumps were enrolled. Secondary attack rate (SAR) was lower in hepatitis A (43.3%) than those for varicella (75.0%) and mumps (65.3%) (p <0.05). Rate of secondary asymptomatic infections was 25.6% in the hepatitis A group, 3.1% in the varicella group, and 16.6% in the mumps group. Positive predictive value (PPV) of the history was 100% for all 3 diseases and negative predictive value (NPV) was 85% for hepatitis A, 80.0% for varicella, and 56.2% for mumps. In conclusion, asymptomatic infections are important in the calculation of SARs in household contacts. PPV and NPV of medical history taken from parents might be useful when deciding the need for vaccination.