Background: Widespread availability of rapid diagnostic testing for respiratory viruses allows more in-depth studies of human parainfluenza viruses (HPIV). Objectives: This study aimed to assess seasonality of HPIV types 1-4, clinical outcomes by HPIV type, and risk factors for illness severity. Patients/Methods: This retrospective study was performed from January 2013 to December 2015 in children and adults with HPIV, detected by multiplex reverse transcription polymerase chain reaction, participating in a community surveillance study of acute respiratory infections (ARIs) in New York City and patients admitted to a tertiary care center in the same neighborhood. Seasonality trends by HPIV type were compared between the community and hospital groups. The associations between HPIV type, demographics, clinical characteristics, and illness severity were assessed. Results: HPIV was detected in 69 (4%) of 1753 community surveillance participants (median age 9.2 years) and 680 hospitalized patients (median age 6.8 years). Seasonality for HPIV types 1-3 agreed with previously described patterns; HPIV-4 occurred annually in late summer and fall. In the community cohort, 22 (32%) participants sought medical care, 9 (13%) reported antibiotic use, and 20 (29%) reported ≥1 day of missed work or school. Among hospitalized patients, 24% had ≥4 chronic conditions. Multivariable ordinal logistic regression demonstrated that increased severity of illness was significantly associated with HPIV-4 and chronic cardiovascular and respiratory conditions in children and with age ≥65 years and chronic respiratory conditions in adults. Conclusions: HPIV-4 presented late summer and early fall annually and was associated with increased severity of illness in hospitalized children.