Chronic fatigue syndrome (CFS) is a medically unexplained illness that is diagnosed on the basis of a clinical case definition; so it probably is an illness with multiple causes producing the same clinical picture. One way of dealing with this heterogeneity is to stratify patients based on illness onset. We hypothesized that either the whole group of CFS patients or that group which developed CFS gradually would show a relation with birth order, while patients who developed CFS suddenly, probably due to a viral illness, would not show such a relation. We hypothesized the birth order effect in the gradual onset group because those patients have more psychological problems, and birth order effects have been shown for psychological characteristics. We compared birth order in our CFS patients to that in a comparison group derived from U.S. demographic data. We found a tendency that did not reach formal statistical significance for a birth order effect in the gradual onset group, but not in either the sudden onset or combined total group. However, the birth order effect we found was due to relatively increased rates of CFS in second-born children; prior birth order studies of personality characteristics have found such effects to be skewed toward first-born children. Thus, our data do support a birth order effect in a subset of patients with CFS. The results of this study should encourage a larger multicenter study to further explore and understand this relation.