A high prevalence of low levels of cobalamin had been found in a survey of multi-ethnic normal individuals in Israel. The purpose of this study was to investigate the incidence of cobalamin deficiency among Israeli couples suffering from infertility. All couples seen at the in vitro fertilization clinic at an urban hospital (Shaare Zedek Medical Center) in Jerusalem for a 6-month period were invited. Mean cobalamin levels were 259.2 pg ml(-1) in males and 275.1 pg ml(-1) in females (normal >200 pg ml(-1)), 35.5% of 172 men and 23.3% of 223 females had cobalamin deficiency (P = 0.01). There were 171 couples with complete demographic questionnaires and cobalamin values for each partner. In 74 couples (43.3%), one partner was cobalamin deficient, with no significant difference between those with unexplained infertility versus those with explained infertility; and in 13 couples, both partners were cobalamin deficient. Thirty-nine per cent of all men with an abnormal semen analysis had cobalamin deficiency, a finding that requires further investigation. This study questions whether higher rates of male infertility in Israel are partially ascribable to cobalamin deficiency. Recommendation for supplementation in both males and females to achieve high-normal levels of cobalamin would be prudent.