Epidemiological studies have shown wide geographical and racial variations in the prevalence and pattern of immunodeficiency diseases. As there is no national registry, very little is known of the prevalence and nature of humoral immunodeficiency in the Arabian peninsula. We report here for the first time the analysis of serum immunoglobulin (Ig) levels in 2000 consecutive patients (age, 1–80 years). They were seen over a period of 6 years and were referred to us from six district hospitals for suspected immunodeficiency, autoimmunity, allergy, or immunoglobulin dyscrasia. Forty-six were found to be immunodeficient, in whom at least one of the Ig class was low; 15 had secondary immunodeficiency. The remaining 31 cases, representing 1.5% of the population studied (giving a prevalence of 1550/100,000 hospital registered patients), were categorized into four primary humoral immunodeficiency groups: these included, in order of frequency, (i) selective IgA deficiency (45%; 700/100,000) (ii) common variable immunodeficiency (CVID) (29%; 450/100,000), (iii) agammaglobulinemia (16%; 250/100,000), and (iv) selective IgG deficiency (10%; 150/100,000). Compared with similar hospital-based surveys in the west the prevalence of humoral immunodeficiency seems to be higher in Arabs; this in part may be related to race and higher rate of consanguinity. Most patients with IgA deficiency had either infection, atopy or autoimmunity. Compared with some other races, agammaglobulinemia (X- and non-X-linked) seems to be more prevalent.