An analysis of peripheral and deep margins of histological clearance around 1539 consecutive basal cell carcinomas excised by conventional surgery showed that 81 lesions (5.3%) were incompletely excised peripherally; 36 lesions (2.3%) were incompletely excised deeply; 13 lesions (0.8%) were incompletely excised peripherally and deeply. Nine hundred and ninety-six lesions (65%) were excised with a peripheral histological clearance margin<5mm (0.1-4.9mm), whereas 1303 lesions (85%) were excised with a deep histological clearance margin<5mm (0.1-4.9mm). Four hundred and eight lesions (27%) had a peripheral histological clearance margin of 5.0-9.9mm, whereas 170 lesions (11%) had a deep histological margin of 5.0-9.9mm. Peripheral histological clearance margins exceeded 10mm in 41 lesions (3%) and deep histological margins exceeded 10mm in 17 lesions (1%). Thus 30% of peripheral histological margins were 5mm or more but only 12% of deep histological margins were 5mm or more. Despite a relative sparing of deep tissue, incomplete excision in depth affected only 36 lesions compared with 81 incomplete peripheral excisions. Peripheral histological clearance was <5mm (0.1-4.9mm) for 55% of temple lesions, 50% of scalp lesions and 43% for limb lesions. In the cosmetically sensitive areas of peri-orbital region, nose, cheek, lip, neck and chin more than 70% of lesions were excised with a peripheral histological margin<5mm. This study of conventional surgical excision of basal cell carcinomas with an incomplete excision rate of 8% has shown that 65% of lesions were excised with <5mm histological clearance peripherally and 85% with <5mm deep clearance. These figures for 'normal tissue sacrifice' are not excessive when compared with those of 'tissue sparing' Mohs' micrographic surgery in which the operator may take a margin of several millimetres of normal tissue in the initial 'slice', or in the subsequent 'safety margin' beyond the eventual tumour free plane. However, peripheral margins did exceed 5mm in more than 30% of lesions of scalp, temple and forehead, and for these sites where even with loupe magnification the tumour edge could be difficult to define, either frozen section control or Mohs' technique, might with benefit be more often used in order to minimise normal tissue sacrifice.