A 52 year old man developed a painful swollen left knee. Clinically there was an effusion and radiographs showed local osteoporosis. Sepsis was suspected but tests proved negative and the symptoms gradually settled. During the course of the following year he developed similar pain migrating from the left foot to the right knee then to the right foot. Investigations showed a right knee effusion and 'punctate' radiographic lucencies in the patellae thought to be erosions. A bone scan demonstrated increased uptake at all four sites and sepsis was once again suspected. Biopsies and culture proved negative and a computed tomographic scan confirmed that the lucencies were due to focal osteoporosis rather than erosions. This case of transient regional osteoporosis illustrates two unusual features of this condition which resulted in diagnostic difficulty with respect to sepsis.