We studied 14 families with familial hypercholesterolaemia (FH) from Central and Southern Tunisia. Twenty-six living homozygotes were identified in these areas of whom 24 homozygotes and 27 of their obligate heterozygote parents are the subject of this report. Ten of the 14 families are unrelated and in 9 of them there were consanguineous marriages. The mean age of homozygotes was 16 for females (range 2.5-40) and 12.5 for males (range 2-34). All the homozygotes had extensive xanthomatosis and showed variable clinical manifestations of coronary heart disease (CHD). Plasma total and LDL cholesterol levels averaged 18 and 16.9 mmol/l, respectively. Mean high density lipoprotein (HDL) cholesterol values were 0.48 mmol/l for males and 0.70 mmol/l for females. The mean age of the obligate heterozygotes was 44 (range 32-62 years) for mothers and 51 (range 35-80 years) for fathers. None of them had tendon xanthomas, not even the oldest, who was aged 80. Only 5 of the 27 obligate heterozygotes had developed CHD (aged 34-58). Plasma cholesterol levels varied more than twofold (4.1-10 mmol/l) and averaged 6.79 and 7.51 mmol/l for males and females, respectively. LDL cholesterol values were below the age- and sex-related 95th percentile from the Lipid Research Clinics Prevalence Study in 46% of male and 30% of female heterozygotes. The frequency of homozygotes was 1:125,000 and the minimum estimated frequency of heterozygotes was 1:165 in Central and Southern Tunisia. Only Afrikaners in South Africa and French Canadians have such high frequencies of FH.