The genetic background of psoriasis is unknown and its mode of inheritance is still controversial. Family studies in psoriasis were based on complex segregation analysis and a special computer programme was prepared. The results of the analysis in 244 families strongly suggest multifactorial inheritance of psoriasis vulgaris. Among the nine hypotheses of rank 1 and rank 2, the lowest value of chi-square, 72.847, was for recessive inheritance, but for the multifactorial model it was extremely low (chi-square=35.980). The estimated heritability was 82%. It might be possible that at least two genetically distinct subpopulations of psoriasis vulgaris exist: one with multifactorial inheritance and a second with multigenic determination, if the disease were due to recessive genes and to one or more dominant factors. The theoretical recurrence risk of psoriasis for the multifactorial model was computed for families with normal parents and for families with one affected parent.