UBVRI observations of 37 QSOs are reported. The sample is considered as a good representation of the bright (V not greater than 17) quasars in the compilation by Hewitt and Burbidge (1980). The continuum has a steeper slope in the ultraviolet as indicated by the correlation between the blue colors and the redshift. That spectral region appears to be decoupled from the remainder. No differences are seen between radio quiet and radio loud quasars in the observed colors. No direct evidence for a synchrotron origin of the continua is present in the data since they are poorly fitted by power laws. A principal component analysis of the data points to the existence of two groups of quasars with differences in their continua as well as in some of the emission lines and in the X-ray luminosities. It is argued that the possible non-thermal component could be diluted by other contributions in particular that of the nebulosities surrounding the quasars.