Summary Parainfluenza viruses types 1, 2 and 3 were found in 2·5%, 0·8% and 1·6% respectively of patients examined in the MRC/PHLS general practice survey and in 2·2%, 0·7% and 2·7% of those in the hospital survey. Type 3 infections were found earlier in life than type 1, while type 2 infections tended to be detected in older children. These viruses were found most frequently in croup and laryngitis but were also common causes of coryza and lower respiratory infections, especially in general practice. The epidemiology and diagnosis of parainfluenza virus infections are discussed briefly.