Psoriasis and systemic lupus erythematosus are common, but their coexistence is thought to be infrequent. Each of these diseases has a broad clinical spectrum, so that diagnosis may not be straightforward. This study aimed to investigate the following immunological parameters in psoriasis patients: (i) direct immunofluorescence (DIF); (ii) antinuclear antibody; (iii) anti-double-stranded DNA (dsDNA); (iv) anti-Ro; and (v) anti-nuclear ribonucleoprotein (nRNP). Of 300 cases, comprising 189 men (62.9%) and 111 women (37.1%), 17 (5.7%; 10 men, seven women) were positive for at least one immunological parameter. Nine of 300 (3%; seven male, two female) biopsy specimens from sun-exposed psoriatic lesions demonstrated bright continuous bands of granular IF along the dermoepidermal junction with immunoreactant immunoglobulin (Ig)G, IgM, C3 and fibrinogen. The intensity of IF at the dermoepidermal junction was graded 3+ and 2+. Three cases demonstrated IgM, two had IgG, two fibrinogen and six cases showed C3. Three cases demonstrated more than one immunoreactant. One case demonstrated C3 at the vessels. No specimen demonstrated IgA deposition. Three hundred sera were obtained from patients with psoriasis, of which five demonstrated elevated antinuclear antibody (ANA) titer; dilution titer varied from 1:80-640. Three had a homogeneous pattern and two had a speckled pattern. None had a peripheral pattern. Five (1.7%) of 300 demonstrated anti-Ro, two had negative ANA, and three were positive ANA, two of which were speckled and the other homogenous. Anti-dsDNA, anti-Sm and anti-nRNP were not detected. Ten patients had positive DIF but negative ANA, while five had positive ANA; all had negative DIF results. Thus, the incidence of psoriasis and lupus erythematosus coexistence is low and a baseline immunological screening test for psoriasis might not prove worthwhile.