Recognizing these weaknesses of the SRS, the Centers for Disease Control and Pre-vention (CDC) began in 1991 to organize a large-linked database study of about500,000 children zero to six years of age in bur Health Management Organizations(HMOS).~ Information on all four cells of a two-by-two table plus potential con-founders are being collected to permit controlled epidemiologic assessment of vaccinesafety concerns, including those raised by this workshop.Despite the shortcomings described, SRS can still provide a modicum of useful post-licensure data on the safety of simultaneous administration of vaccines. If prior to theintroduction of the new procedure or new combined vaccine baseline SRS experiencewith at least one of the antigens is available and the reporting behavior to the SRS re-mains otherwise unchanged, a crude ecologic analysis can be done to see if there is