The purpose of this study was to examine the possible influence of the phases of the menstrual cycle on dermal reactivity to skin-prick testing. We studied 15 atopic, menstruating women with seasonal rhinoconjunctivitis and/or asthma, with known sensitivity to olive and parietaria (mean age 25.2 years) and 15 non-atopic, healthy, female controls (mean age 24.7 years). Skin-prick tests with histamine, morphine, and in the atopic group with parietaria/and/or olive, were repeated three times during the same menstrual cycle, corresponding to bleeding (day 1-4), midcycle (day 12-16) and the late progesterone phase (day 24-28). None of the patients had received oral antihistamines or exogenous hormones for at least 1 month prior to testing. Results indicate a significant increase in weal-and-flare size to histamine, morphine, and parietaria on days 12-16 of the cycle, corresponding to ovulation and peak oestrogen levels. This was observed in both atopic and non-atopic women. Differences in skin reactivity to histamine and morphine between the groups were not significant. Therefore, in women, the phase of the menstrual cycle is another factor that may influence skin-test results.