The case of a 59-year-old female with a longstanding hemiplegia who developed unilateral psoriatic arthritis and unilateral psoriasis on the non-hemiplegic side only is reported. Investigations showed an erythrocyte sedimentation rate of 58 mm/h, with all classes of rheumatoid factor negative in serum and synovial fluid (SF). An inflammatory infiltrate was demonstrated in both knee joints despite the absence of clinical involvement on the hemiplegic side. In addition, while neurofilament and substance P (SP) immunoreactivity was demonstrated in both synovial specimens, conspicuously less SP was present in the clinically involved synovial membrane. Finally, SF levels of SP and IL1 beta were raised in the clinically involved knee only.