Epidemiological studies have shown wide geographical and racial variation in the prevalence and patterns of immunodeficiency disorders. To determine the frequency of primary immunodeficiencies (PID) in Iran, the Iranian Primary Immunodeficiency Registry (IPIDR) was organized in 1999. We extracted the patient's data, by using a uniform questionnaire from their hospital records. The diagnosis of patients was based on WHO criteria. By now, 440 patients with PID, who were observed during a period of 20 years, have been registered in our registry. Among these patients, the following frequencies were found: predominantly antibody deficiency in 45.9% of patients (n = 202), phagocytic disorders in 29.09% (n = 128), T-cell disorders in 24.31% (n = 107), and complement deficiencies in 0.68% (n = 3). Common variable immunodeficiency was the most frequent disorder (n = 98), followed by chronic granulomatous disease (n = 86), ataxia telangiectasia (n = 48), x-linked agammaglobulinemia (n = 45), selective IgA deficiency (n = 42), combined immunodeficiency (n = 15), and severe combined immunodeficiency (n = 14). This study revealed that antibody deficiencies is the most frequently diagnosed primary immunodeficiency disorder in our patients, which is similar to that observed in other registries. A comparative study shows some differences between our results and other registries.