Generally positive correlations between different ability tests provide the evidence for a factor of "general intelligence" or Spearman's g. Though a possible neural substrate for g is suggested by executive impairments following frontal lobe lesions, preserved IQs in some frontal patients have been taken as strong evidence against this interpretation. We show that such results depend on how g is measured. Patients with superior IQs on the most clinically popular test--the Wechsler Adult Intelligence Scale--show impairments of 20-60 points on conventionally measured fluid intelligence or novel problem solving. On psychometric grounds, it is fluid intelligence that is most closely related to Spearman's g. The data suggest that g may in large part be a reflection of frontal functions.