Total body calcium was measured using in vivo neutron activation analysis in 156 patients with rheumatic diseases at six-month intervals. Evidence of seasonal variation was sought by relating deviation from a linear change to the month in which the measurement was made. A cyclic regression fitted to the data had an amplitude of 0.25% but a significance of only P=0.08. The calcium balance studies of Malm (see Scand. J. Clin. Lab. Inv., vol.10, supplement 36, p.1-290, 1958) were re-analysed to quantify the seasonal variation he had demonstrated in one group of men. The deduced variations of body calcium were highly significant, with a phase and amplitude very similar to the regression of the authors' data. Both analyses differed from two previous reports of seasonal variations of part-body bone mineral of much greater amplitude and different phase.