HL-A typing was performed in a series of 22 patients with persistent palmoplantar pustulosis, 14 patients with palmoplantar pustulosis and psoriasis, 15 patients with generalized pustular psoriasis and 45 patients with uncomplicated psoriasis. None of the patients with persistent palmoplantar pustulosis alone had relatives with psoriasis, which was a common finding in the other groups. The HL-A antigens 13 and 17 were markedly increased in the group of uncomplicated psoriasis. In the group of patients with persistent palmoplantar pustulosis alone neither of these antigens was increased, as compared with a control population. Four patients with persistent palmoplantar pustulosis and psoriasis had HL-A 17 or 13 and two patients with generalized pustular psoriasis had HL-A 17. In the latter group HL-A 27 occurred in eight cases and correlated with the presence of sacro-iliitis. It was concluded that persistent palmoplantar pustulosis is an entity genetically distinct from psoriasis even if psoriatic patients might have an increased tendency to develop this type of pustular reaction.