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Abstract Background Difference in severity of dengue outbreaks has been related to virus serotype, genotype and clades within genotypes. Till the 1980 s, India and Sri Lanka reported low number of dengue hemorrhagic fever (DHF) cases despite circulation of all four serotypes of dengue virus (DENV). Since the 1990 s the occurrence of DHF has increased. The increase has been attributed to changes in virus lineage especially with regard to DENV-2 and DENV-3. DENV-1 has been associated with dengue fever (DF) outbreaks and DENV-4 reports have been rare. The emergence of DENV-4 was reported recently in 2003 in Delhi and in 2007 in Hyderabad. The last report of DENV-4 from Maharashtra was in 1975 from Amalner. Results We report on the detection of DENV-4 in Pune, Maharashtra after an absence of almost 30 years. Two cases were detected in 2009-10, serotyped by multiplex reverse transcriptase polymerase chain reaction (RT-PCR). Both the cases were recorded as severe dengue (Category 3) requiring intensive care unit (ICU) level of treatment. Depending on the hemagglutination inhibiting (HI) antibody titres the 2009 case was characterized as a primary infection and the 2010 case as a secondary infection. Both the cases presented plasma leakage and neither showed any kind of haemorrhage. The 2009 case survived while the 2010 case was fatal. An isolate was obtained from the 2009 case. Based on envelope (E) gene sequence analysis, the virus belonged to genotype I of DENV-4, and clustered with isolates from India and Sri Lanka and was distant from the isolates from Thailand. The nucleotide and amino acid diversity of the E gene of the Indian isolates increased from 1996 to 2007 to 2009 in context of the E gene sequences of other isolates belonging to genotype I. Conclusion The increasing diversity in the circulating DENV-4 calls for close monitoring of the DENV-4 serotype.