Antinuclear antibodies (ANA) frequently arise in the sera of children with connective tissue disease and is used in the diagnosis of these diseases. Therefore it is also important to know the prevalence of ANA in normal children. The main objective of the present study was to determine the prevalence of antinuclear antibody (ANA) in healthy children. Ninety-nine serum samples from a serum bank and 108 samples from patients who had attended elective surgery and whose blood had been withdrawn for other investigations, were tested for ANA by indirect immunofluorescence method using HEp-2 cells as substrate. Sera from 52 children with SLE were also tested during the same period. It was found that antinuclear antibodies were present in 32 (15%) of the 207 sera of healthy children at a dilution of 1:40 or higher. ANA were positive in 9% at a serum dilution of 1:40, in 3% at 1:80 and in 3% at 1:160. The patterns of immunofluorescence staining were as follows: homogeneous in 46.7%, speckled in 20%, and nucleolar in 10%. In SLE patients, ANA were positive in 91%; 13% at a serum dilution of 1:40, 7% at 1:80, 20% at 1:160, 15% at 1:320, 9% at 1:640, 20% at 1:1,280 and 9% at > or = 1:2,560. It was concluded that the prevalence of positive ANA using the HEp-2 cells as substrate was 15% in healthy children at dilutions of 1:40 or higher. Using the cutoff serum dilution of 1:40, the sensitivity of this test was 91%, the specificity was 85%, the positive predictive value was 57% and the negative predictive value was 97%.