Toxic shock syndrome (TSS) is caussed by toxic shock syndrome toxin-1 (TSST-1) of Staphylococcus aureus. We studied the incidence of TSST-1 production by the clinical isolates of S. aureus, in order to clarify the possibility of TSS caused by S. aureus. One hundred and seventeen clinical isolates of S. aureus were tested. Of 117 strains, 74 were methicillin-cephem resistant S. aureus (MRSA), and 43 were methicillin-cephem sensitive S. aureus (MSSA). TSST-1 production and penicillinase (PCase) activity were measured by means of reversed passive Latex aggulutination method and acidmetric assay, respectively. The incidences of TSST-1 production by PCase positive MRSA, PCase negative MRSA, PCase positive MSSA and PCase negative MSSA were 92%(55/60), 78%(11/14), 21%(7/33) and 0%(0/10), respectively. TSS occurred in a patient with chronic complicated pyelonephritis after endopyelotomy for hydrone-phrosis. In this patient, PCase positive MRSA which produced TSST-1 was isolated from urinary tract, and he was cured after administration of arbekacin. These results indicsate that methicillin-cephem resistance and PCase production are the risk factors for TSST-1 production in S. aureus.