The influence of Picolax bowel preparation on the risk of explosive colonic gas was studied in 35 patients. The mean concentration of explosive hydrogen was 1% (0-8%) and methane was 3% (0-17.3%). Three samples contained > 4% potentially combustible hydrogen concentrations whilst 13 samples showed potentially combustible methane levels ( > 5%). Fifteen of 35 patients (43%) had potentially or actually explosive gas mixtures: 12 of 19 (63%) patients with an unsatisfactory bowel preparation compared with 3 of 16 (19%) with a satisfactory preparation (p < 0.01). The risk of explosive colonic gas is determined largely by the quality of bowel preparation. Picolax alone does not eliminate the risk of explosion during colonoscopic polypectomy or surgical diathermy at laparotomy or laparoscopy if the bowel is opened.