Dengue is an overlooked tropical disease for which billions of people are at risk. The disease, caused by a Flavivirus with four distinct serotypes, is transmitted primarily by urbanized Aedes mosquito species. The infection leads to a spectrum of clinical manifestations, with the majority being asymptomatic. Primary dengue and, to a greater extent, subsequent infection, mainly secondary dengue infection, are associated with increased severity. Increased global travel and recreational tourism expose naïve individuals to dengue, the most common arboviral infections in travelers. We describe a cluster of possible primary acute dengue infections in a group of 12 individuals who presented to Bangkok Hospital for Tropical Diseases in 2017. Infection was confirmed by dengue NS1 antigen and multiplex real-time RT-PCR. Nine individuals required hospitalization, and four developed dengue warning signs. The mean arterial pressure was significantly lower in the group with dengue warning signs. The period from the day of arrival in Thailand and the first day of symptoms was significantly shorter in adolescents with warning signs. Leukocytes, neutrophils, and platelets declined significantly at defervescence and were negatively correlated with day of illness. Six clinical isolates were identified as dengue serotype-1, with identical sequences suggesting that these patients were infected with the same virus.