Four patients meeting the "classical" criteria for spontaneous pyomyositis are reported. No local cause was found. Risk factors included diabetes mellitus (2 patients), hemopathy (one patient), and alcohol abuse (one patient). Causative organisms (Staphylococcus aureus in 3 cases and Salmonella sp in one case) were recovered from blood cultures, an unusual occurrence. CT scan studies ensured the diagnosis in every case. An additional case of pyomyositis due to Staphylococcus aureus illustrates the difficulties in the definition of disease. This patient, whose muscle lesions were remarkably well visualized by MRI with injection of gadolinium, developed infection of the sacro-iliac joint adjacent to the muscular focus of infection. This patient may have had either "primary" pyomyositis with spread to the adjacent joint or "secondary" pyomyositis caused by the joint infection which was recognized only later. Advances in medical imaging techniques suggest that the nosology of pyomyositis should be broadened using this terminology. This would underscore the unique characteristics of "classical", "primary" pyomyositis and emphasize imaging, diagnostic and therapeutic facets of the disease which are shared by both entities.