Previous publications suggest that in patients with systemic lupus erythematosus (SLE), rheumatoid factor (RF) may be "protective" against nephritis. In our study of 662 patients with SLE, we found that persistent, rheumatoid-like arthritis showed a much stronger inverse correlation with nephritis than RF. Of 186 such patients, 59 developed clinically evident nephritis (32%) compared to 263 of the other 476 patients (55%) (p less than 10(-7). RF showed only a weak inverse relationship to nephritis (p = 0.064). We conclude that the presence of persistent rheumatoid-like arthritis in patients with SLE identifies a clinical subset of patients who are less likely to develop nephritis than those with no arthralgia, no objective arthritis or only episodic arthritis. We hypothesize that such patients represent a genetically determined subset among patients with SLE and that perhaps they are more likely to bear the HLA-DR4 allele.