During 1982-1983, susceptibility to varicella-zoster virus (VZV) in 224 Thai subjects at high risk for varicella infection was studied. The immune adherence hemagglutination (IAHA) and VZ skin test were carried out to determine VZV immunity in immunocompromised children and young adults. The history of varicella and herpes zoster from each subject was recorded. The mean +/- SD age in children and young adults were 7.3 +/- 2.8 and 19.6 +/- 1.2. Negative IAHA test was found in 74.2% of 62 children and 35.2% of 162 young adults. The increase in immune individuals was demonstrated with advancing age. Response to VZ skin test showed positive results in 79 of 162 (48.8%) young adults. The seronegativity was related to the negative VZ skin test (p less than 0.001, X2 test). Regardless of antibody detection or VZ skin test, 47 of 162 (29%) young adults were susceptible. According to the positive history of varicella and of herpes zoster obtained from 95 young adults, 80% had developed varicella during 1 to 10 years of age and 8.8% had positive history of herpes zoster. The findings suggest that the IAHA and VZ skin test should be used together for assessing VZ immunity. Varicella vaccination is highly recommended for susceptible persons who may develop severe illness.