The reliability of the grip force instrument, Grippit was tested on 51 right-handed women of which 18 were healthy, 19 had rheumatoid arthritis (RA) and 14 had fibromyalgia (FM). Normative data was obtained from 169 healthy subjects. Results indicate that the reliability of Grippit was high in healthy and rheumatoid arthritic women and satisfactory in women with fibromyalgia. All patients showed greatly reduced grip force (RA had on average 21% and FM 40% of the control values) when compared to healthy women. Healthy women had on average 54% of men's grip force. The ratio between average force over 10 seconds and peak force was 73% for RA women, 69% for FM women, 83% for healthy women and 85% for healthy men.