Data from the national Health Interview Survey for 1957-72 show that females have higher age-standardized rates of acute conditions, chronic conditions, and disability due to acute conditions, compared to males. More males, however, suffer limitations of activity or mobility due to chronic conditions, Females' excess morbidity for acute conditions persists when reproduction-related events are excluded. From 1957-72, females' disadvantage for acute conditions continues unchanged. But for chronic conditions, males are "catching up" to females in overall prevalence, and their conditions are becoming more severe relative to females'. These trends are compatible with trends in sex mortality differences over the same period. Four types of explanation are advanced to account for females' excess morbidity: interview behavior, illness behavior, acquired health risks, and inherited health risks. Empirical evidence suggests the first two factors inflate female rates compared to males; the last two do the opposite. It is hypothesized that excess female morbidity is due primarily to social and psychological factors, and a procedure to test this is stated.