The first-degree family members of patients with Behcet's syndrome were investigated to determine (1) whether there was a familial aggregation of Behcet's syndrome or of its component features, (2) whether there was an increased incidence of sacro-iliitis in these families, and (3) whether a link exists between Behcet's syndrome and spondarthritides in first-degree relatives. Of the 226 relatives and patients seen, 21 patients with definite Behcet's syndrome and their 71 relatives were analysed in some detail. Orogenital ulceration was found in 2 mothers of probands; 1 patient had psoriasis and Behcet's but neither disease featured in the pedigree. Another family showed numerous manifestations of psoriasis and psoriatic arthritis, 1 member having severe sacro-iliitis but the proband had Behcet's syndrome only. Family trees including HLA pedigrees, where known, are presented. Spouses suffered few of the complaints found in patients, and to a less extent in relatives, but showed a high incidence of backache, perhaps of psychogenic origen.