Background Pathogenic CD4+CD28null cells are characterized by inflammatory cytokine synthesis and tropism to the inflamed tissues. Recent studies showed the involvement of CD28null T cells in a severe clinical outcome of lupus. However, their role in moderately active disease is still unresolved. Methods We examined the levels of circulating CD4+CD28null cells and CD8+CD28null suppressor T cells. We also compared the CD4+CD28null and CD4+CD28+ T-cell functional properties, including the expression of interferon gamma (IFN-γ) and Ki67 among systemic lupus erythematosus (SLE) patients ( n = 20) and healthy controls ( n = 20). All the patients were under immunosuppressive treatment and exhibited moderate SLE activity (median SLE Disease Activity Index (SLEDAI) = 6). Results In patients, we found elevated CD4+CD28null and unchanged levels of suppressor CD8+CD28null T cells. There was no difference between patients and controls in IFN-γ and Ki67-expressing CD4+, CD4+CD28+, and CD4+CD28null T cells, except for higher IFN-γ levels in CD4+CD28+ T cells in SLE. In each studied group, we observed a higher preponderance of IFN-γ- and Ki67-expressing cells among CD4+CD28null T cells and lower levels of IFN-γ in CD4+CD28null T cells compared to the CD28+ subset. Similarly, Ki67 intensity was decreased in healthy CD4+CD28null cells, whereas in patients, comparably high expression was observed in both subsets. IFN-γ intensity in CD4+CD28null T cells correlated with SLEDAI. Conclusion SLE with a moderately active clinical course is characterized by peripheral blood expansion of CD4+CD28null T cells and a normal abundance of suppressor CD8+CD28null T cells. The demonstration that these pathogenic CD4+ T cells, despite the lack of CD28, maintain the ability to produce pro-inflammatory IFN-γ positively correlated with disease activity as well as relatively high proliferative capacity may suggest their potentially predictive role in SLE flares.