High resolution maps of the H II regions DR 21, W 49A and W 3 have been made with the Cambridge One-Mile radio telescope at 5 GHz with a halfpower beamwidth of 6.5 arc sec. Compact regions are clearly visible with sizes typically in the range 0.1–1.0 pc. The peaks of two of the condensations in W 49A are very close to OH emission regions, as is that of G 133.9 + 1.1 in W 3, which has a turnover frequency of 18 GHz. Comparison of radio data with optical isophotes of W 3 indicates a range of at least 12 magnitudes in the obscuration for different parts of the source. The most obscured part contains a group of four condensations one of which has a clear ring structure ½ pc in diameter. By combining data from W 3, W 49, W 58 and DR 21 it is shown that the condensations are probably surrounded by neutral hydrogen and that some, at least, must contain more than one of the earliest type stars known. The time scale of evolution and the connection between H II condensations and subgroups of OB stars in associations is discussed. The discovery of a small continuum radio source close to the W 75(N) OH source is also described.