The QRS duration (QRSD) on a digital 12 simultaneous lead ECG was measured by a commercially available recording cart (Marquette MACII 12SL) in 1,254 white male safety workers (ages 19-65, mean 34). All had a negative history (including drugs known to affect the cardiovascular or pulmonary systems), a negative family history (in immediate family members before age 55), no physical findings suggestive of heart disease, a normal blood chemistry profile, pulmonary function tests, and symptom limited bicycle exercise test. The frontal QRS axis was between -30 and -65 in 22 of 1,254 (1.8%). Twenty-seven of 1,254 (2.1%) had QRSD greater than or equal to 120 ms-14 of these had normal morphology; 2 had RBB; 3 had atypical RBB; 5 had R' in V1, V2; 2 had WPW; and 1 had Superior Fascicular Block. Sixty-three (5%) had a QRSD greater than or equal to 112 and less than or equal to 116 ms-36 of this group had normal morphology; 1 had typical RBBB; and 26 had R' V1, V2 (considered a normal variant as it occurred in 360 of 1,164 remaining with QRSD less than or equal to 108). In 1,224 white men with normal QRS morphologies and frontal axis (-25 to 100), the 98% upper and lower bounds of QRSD with the 12SL algorithm, like that seen in BSMs, was 80-116 ms, peak 96 ms.(ABSTRACT TRUNCATED AT 250 WORDS)