We present 5 patients in whom thallium poisoning (1) mimicked systemic lupus erythematosus (SLE) with positive antinuclear antibodies; (2) caused a SLE-like illness with positive antinuclear antibodies and rheumatoid factor that was followed by a persistent chronic polyarthritis; (3) mimicked SLE with negative antinuclear antibodies and no residual disease; (4) caused arthralgia, Raynaud's phenomenon and palmar erythema with negative antinuclear antibodies followed by keratoconjunctivitis sicca; and (5) triggered the onset of SLE with diffuse proliferative glomerulonephritis. Although the notion exists that thallium poisoning may simulate seronegative SLE, these associations between connective tissue diseases and thallium poisoning have not been previously recorded.