Parainfluenza virus infections (PIV) were evaluated in patients with mild and severe infections through real time PCR. One thousand and sixty-seven samples were collected from subjects as follows: 233 adult renal transplanted outpatients, 129 children with congenital heart disease, 381 with adult hematopoietic stem cell patients and 324 hospitalized patients suspected of influenza A (H1N1) pdm09 infection. PIV was detected in 74 (6.9%) samples. VPI-3 was the most frequent (60.8%) and a higher risk was observed for older adults (p = 0.018) and for those who were hematopoietic stem cell transplanted. Further studies are needed to understand the VPI role in patients' at risk for developing serious illness.