Introduction: Chikungunya infection is an arbovirus disease generally considered mild, characterized by fever accompanied by joint pain. It first appeared in Réunion in March 2005 and caused an unprecedented epidemic (24,000 cases between April 2005 and March 2006; incidence: 30%). In late 2005, hospital physicians reported cases of mother-to-newborn transmission and severe clinical forms differing from those classically described. A surveillance system was established in February 2006 to identify and describe these emerging hospital cases. Method: A retrospective and prospective survey of hospitalized cases starting in April 2005 was conducted in the island’s four hospitals. A mother-to-newborn case was defined as any newborn under 10 days of age with a biologically confirmed chikungunya infection. A hospital-based emerging case was defined as any patient aged 10 days or older hospitalized with a biologically confirmed chikungunya infection presenting symptoms other than fever and arthralgia. Surveillance data collected between April 2005 and March 2006 were analyzed. Results: During the surveillance period, 878 hospital-acquired cases were identified: 44 maternal-neonatal cases and 834 hospital-acquired cases, of which 247 (30%) were severe. Sixty-eight (8%) of these patients died. These cases accounted for 0.36% of chikungunya cases in the population. Dermatological (347 [40%] cases), gastrointestinal (333 [38%] cases), and neurological (327 [37%] cases) manifestations were the most common. Hospitalized cases occurred mainly among vulnerable individuals (young children, the elderly, and patients with comorbidities). Discussion: The hospital surveillance system implemented on an emergency basis made it possible to document rare but potentially severe forms of chikungunya. The role of the virus in the occurrence of these manifestations must be clarified through clinical research studies. The results of this surveillance indicate the need to prioritize prevention recommendations for the most vulnerable individuals during a chikungunya epidemic. (R.A.)