This paper describes the comparability of counts in the locus coeruleus made from serial sections and those made from a single section at the point of greatest density of neurons. Samples of the locus coeruleus neuronal population derived from single section counts though not exactly comparable, are of comparable utility to those obtained from more laborious total neuronal counts. The simpler method was used to examine the hypothesis that there are subtypes of senile dementia of the Alzheimer type. Separation into two groups was achieved when independent variables of cortical neuronal counts and tangle estimates were used. This finding adds to the growing evidence that Alzheimer's disease of the senile type is not a unitary disorder.