Hospital effluent and connected waste water treatment plant (WWTP) influent and effluent were sampled daily to determine the levels and inter-day variations of three naturally occurring steroid estrogens: estrone, 17β-estradiol, estriol, and synthetic 17α-ethinylestradiol. After solid phase extraction, interferences were removed with a silica gel clean-up step and the samples analysed using gas chromatography with mass selective detection (GC-MSD). The determined inter-day concentrations in hospital effluent were between 8.6 to 31.3 ng L(-1) for estrone, <LOD (limit of detection) to 4.2 ng L(-1) for 17β-estradiol and 6.4 to 385.5 ng L(-1) for estriol. In the WWTP influent concentrations were 18.9 to 49.7 ng L(-1) for estrone, 2.4 to 12.7 ng L(-1), for 17β-estradiol and <LOQ (limit of quantitation) to 63.9 ng L(-1) for estriol. Reduced levels were found in the WWTP effluent: <7.1 ng L(-1) for estrone, <LOQ for 17β-estradiol and <5.2 ng L(-1) for estriol. 17α-ethinylestradiol was detected in only one influent sample. Calculated estradiol equivalents (EEQ) were 33.4, 22.4, 1.7 ng (EEQ) L(-1) in the hospital effluent, WWTP influent and WWTP effluent, respectively. Interestingly, the estrone: 17β-estradiol:estriol ratio in the hospital effluent (1:0.1:9.4) is comparable to that found in the urine of pregnant women (1:0.3:20) indicating the most likely source of steroid estrogens. In WWTP influent the ratio was similar to that found in the non-pregnant population. Our result recognise estriol as being one of the most important steroid estrogens, accounting for up to 92% of the total EEQ present in hospital samples and 37% and 46% in WWTP influent and effluent samples, respectively. The study reveals how concentrations of steroid estrogens vary on a daily basis and concludes that careful sampling strategies must be adopted when making a risk assessment. In addition, the low potency steroid estrogens that contribute towards overall estrogenicity of the sample, e.g. estriol, should be incorporated into environmental monitoring programs.