In a previous publication (de Blcourt, Polman, and de Blcourt-Meindersma, 1961), we drew attention to the familial occurrence of rheumatoid arthritis and ankylosing spondylitis, each of which shows a special hereditary factor, but we did not report on the presence or absence of the so-called rheumatic factor in probands and members of the family.The "epidemiology" of various rheumatic conditions, including rheumatoid arthritis, has been the subject of several international conferences,* and it has been stated by various workers (for instance, Lawrence and Ball, 1958;Bremner, Alexander, and Duthie, 1959; Ziff, Schmid, Lewis, and Tanner, 1958; Ziff, 1961; Karten, DiTata, McEwan, and Tanner, 1962) that more persons with rheumatoid arthritis and/or positive serological reactions (showing the presence of the so-called rheumatoid factor) are found among the near relatives of sero-positive rheumatoid arthritics than among those of sero-negative arthritics.In near relatives of sero-positive rheumatoid arthritics, Lawrence and Ball (1958) relatives of sero-positive and sero-negative rheumatoid arthritics, but there was a slight tendency to find more sero-positive relatives in a group of seropositive probands.Lawrence and Ball (1958) pointed out that the frequency of sero-positivity in relatives of seropositive probands was greater than in the normal population, whether clinical rheumatoid arthritis was present or not. In families of sero-negative patients with rheumatoid arthritis, clinical rheumatoid arthritis was more frequent, but sero-positivity was not. They concluded that possibly a seronegative form and a sero-positive form ofrheumatoid arthritis were involved.In the present study we have tried to make a small contribution to the question: Is this hereditary aetiological factor in rheumatoid arthritis restricted (or partly restricted) to sero-positive rheumatoid arthritis ? Material We examined the near relatives (parents, siblings, and children aged 15 years and over) and also the spouses of 31 sero-positive and 31 sero-negative probands with definite or classical rheumatoid arthritis (A.R.A. criteria). The probands were regarded as sero-positive if both the Waaler-Rose test and the latex-fixation test (Singer and Plotz, 1956) were positive at dilutions 1: 32 and 1: 160 respectively. The probands were aged 45 to 60 years, each group included twelve males and nineteen females, and they all came from the "Rheumatism Unit" at Groningen.In the sero-positive group of probands there were 226 near relatives alive, and in the sero-negative group 233. Of the 459 relatives, seventeen failed to co-operate, but 442 were examined, a completion rate of 96-5 per cent. All the probands were or had been married; 54 spouses were examined, two refused, and six were dead.The serological tests were carried out and a physical examination was made by one of us (a rheumatologist) at the out-patient clinic or, in some cases, at the relative's home.
