Ultrastructure of human respiratory cilia was studied in ultrathin serial sections and in random fields of sections. The apex of the cilium had dense gray material under the plasma membrane, where the “central” pair of tubules (often slightly eccentrically placed) and the single peripheral tubules ended. Thereafter the cilium thickened gradually about 1 µm below the apex and after that remained reasonably constant. At about 0.5 µm below the apex was a transformation zone (0.1 µm long), where both double and single peripheral tubules were seen. Dynein arms were not seen in the transformation zone but occurred approximately 0.1 µm below it. In a cilium, one peripheral double tubule was absent in the transformation zone but was seen about 0.2 µm below. This resulted in an artificial 8 + 2 pattern in a few sections. In cross sections at levels with peripheral double tubules the mean of the minor axes of the cilia was 0.198 ± 0.009 µm (± SD) calculated from the serial sections of one patient. In 20 patients the variation was greater and the mean was higher: 0.210 ±0.042 µm (± SD).